Epic weapons that cannot bypass DR x/Epic

No, they can't be enchanted, because they aren't masterwork and can never be, and all magic weapons must be crafted with a masterwork base.

Creating Magic Weapons

To create a magic weapon, a character needs a heat source and some iron, wood, or leatherworking tools. She also needs a supply of materials, the most obvious being the weapon or the pieces of the weapon to be assembled. Only a masterwork weapon can become a magic weapon, and the masterwork cost is added to the total cost to determine final market value.

It is quite plain that they are not masterwork by default, given the lack of +1 to attack rolls. So let's suppose we want to upgrade them. But it is impossible for an existing unarmed strike to be made masterwork, per this clause in Weapons:

You can’t add the masterwork quality to a weapon after it is created; it must be crafted as a masterwork weapon (see the Craft skill).

All right, let's try making a new one that's masterwork. This fails because unarmed strikes are not manufactured weapons, and you can't manufacture a weapon with the Craft skill if it's specifically not manufactured at all.

Glossary — manufactured weapons:

This category also includes […] in essence, any weapon that is not intrinsic to the creature.

Monk's class features do not help with this, since Craft is not an effect and neither enhances nor improves anything:

A monk’s unarmed strike is treated both as a manufactured weapon and a natural weapon for the purpose of spells and effects that enhance or improve either manufactured weapons or natural weapons.

How about buying one? You might actually be able to buy an ordinary unarmed strike, since they're listed in the Equipment section in the Weapons table, and you needn't pay anything for it*. Unfortunately, per the quote on MW weapons, a weapon must be crafted as masterwork to be MW, and per the preceding this is not possible even for NPCs. There might be an exception, as there seems to be for magic oils, which you can buy in shops but not craft, except that nowhere is "MW unarmed strike" listed for sale.

Magic fang and company do not make a weapon into a "magic weapon"; rather, they give a weapon a +1 enhancement bonus (and the ability to bypass DR /magic because of that), which is not technically the same thing. And while all magic weapons are masterwork by definition as well as by prerequisite, magic fang and friends do not have to be interpreted as giving a weapon retroactive masterwork status, for essentially the same reason, which is fortunate as otherwise the rules would create some bizarre and abusive situations†.

Magic Fang

Magic fang gives one natural weapon of the subject a +1 enhancement bonus on attack and damage rolls.

Magic Weapon

Magic weapon gives a weapon a +1 enhancement bonus on attack and damage rolls. (An enhancement bonus does not stack with a masterwork weapon’s +1 bonus on attack rolls.) You can’t cast this spell on a natural weapon, such as an unarmed strike (instead, see magic fang). A monk’s unarmed strike is considered a weapon, and thus it can be enhanced by this spell.

Magic Weapons

All magic weapons are also masterwork weapons, but their masterwork bonus on attack rolls does not stack with their enhancement bonus on attack rolls.

Special Abilities

Some monsters are vulnerable to magic weapons. Any weapon with at least a +1 magical enhancement bonus on attack and damage rolls overcomes the damage reduction of these monsters.

Note the phrasing of the last quote; they are described as vulnerable to magic weapons, but that's not the rules text, just the plain description, since vulnerability is not defined (except in the incompatible context of energy vulnerability; it should be clear that DR 5/magic does not mean that a +1 longsword does 50% extra damage as well as overcoming DR). The actual rules say that any weapon with a +1 magical enhancement bonus (whether or not the weapon is, per se, a "magic weapon" itself otherwise) overcomes that DR.

(emphasis added to quotes)

* This is, to my mind, patently absurd, but hey, RAW. Monks won't be proficient with those unarmed strikes, but hey, RAW.
† For example, casting magic weapon to increase the price of a club from 0gp to 300gp (permanently? who knows!) and selling it. Or casting magic weapon on any plain weapon to make it masterwork for future crafting. There is a Pathfinder spell for this purpose. It does nothing else‡, is a level higher, costs as much as crafting a MW weapon in the first place, and takes an hour to cast. Allowing magic weapon to do this for free in a standard action is ridiculous.
‡ It has this excellent provision: "If the target object has no masterwork equivalent, the spell has no effect." Doesn't work on unarmed strikes, as they appear to have no MW equivalent and are not objects.

Unless a magic weapon special ability says so, a weapon special ability isn't bestowed upon a weapon's ammunition

Magic items—like feats, spells, and other effects—do only what they say they do unless the DM says otherwise. Thus if a weapon special ability does not say that the weapon special ability is bestowed upon its ammunition, the weapon with that weapon special ability does not bestow that weapon special ability upon its ammunition.

However, that doesn't prevent the existence of weapons like the +1 dancing longbow, the +1 wounding crossbow, or the +1 spell storing sling, but those weapon special abilities when applied to those weapons apply only to the weapons themselves rather than to the ammunition they launch; usually this means if wielders want those special abilities to take effect the weapons must be used as improvised weapons (PH 113).

Addressing these examples, a loosed +1 dancing longbow smacks folks on its own as if the wielder were using it as an improvised weapon, a wizard who successfully bashes a bear with his +1 wounding crossbow does so using the crossbow as an improvised weapon but nonetheless magically wounds the bear, and when the halfling outrider slaps his mount with his +1 spell storing sling the outrider can take a free action to cast on his mount the spell stored within that sling. (Slapping even an allied creature with a +1 spell storing sling is still using the sling as an improvised weapon.)

The Dungeon Master's Guide's tables don't tell the whole story

When making a case for limiting weapon special abilities to certain weapons, proponents often point to Table 7–14: Melee Weapon Special Abilities and Table 7–15: Ranged Weapon Special Abilities and say that if a weapon special ability doesn't appear on the appropriate table that weapon special ability can't be placed on weapons of that type. However, the reason these tables are present is for

Random Generation: To generate magic weapons randomly, first roll on Table 7–9: Weapons, and then roll on Table 7–10: Weapon Type Determination. Use Table 7–14: Melee Weapon Special Abilities, Table 7–15: Ranged Weapon Special Abilities, or Table 7–16: Specific Weapons if indicated by the roll on Table 7–9. (DMG 223)

So if not generating magic weapons randomly, the tables go unused.

What the tables are actually doing is preventing the DM from being a jerk. See, when the DM is randomly generating a creature's treasure, he rolls on Table 7–9: Weapons to determine the weapon's enhancement bonus, then on Table 7–10: Weapon Type Determination (yielding a 70% chance of a melee weapon, 20% chance of a ranged weapon, and a 10% chance of an uncommon weapon); then on the kind-of-weapon Tables 7–11, 7–12, or 7–13; then, finally, on the appropriate melee weapon special ability or ranged weapon special ability table (Table 7–14 or 7–15). Because Table 7–13: Common Ranged Weapons includes both ammunition and the weapons using ammunition, every weapon special ability on Table 7–15: Ranged Weapon Special Abilities must be compatible with both ranged weapons and their ammunition.¹

Were there but a lone table for randomly determining weapon special abilities, the DM would either roll until getting an effective combination or laugh maniacally as he generated stupid weapons suitable only as vendor trash. For example, the tables prevent the DM from randomly generating loot piles containing +1 disruption longbows (which unleash their destructive power against undead when such longbows are used as improvised weapons) or +1 speed arrows (which give an extra attack when used as improvised weapons). The tables for random treasure generation are actually kind of spiffy that way.²

Curious Case #1: Defending

The weapon special ability defending says

A defending weapon allows the wielder to transfer some or all of the sword’s enhancement bonus to his AC as a bonus that stacks with all others. As a free action, the wielder chooses how to allocate the weapon’s enhancement bonus at the start of his turn before using the weapon, and the effect to AC lasts until his next turn. (DMG 224)

Emphasis mine. Other defending weapons notwithstanding (e.g. the dagger of defense (AE 104) (58,302 gp; 1 lb.); the dwarven waraxe Gharriakha, the Hearthwarden (RS 165) (61,830 gp; 8 lbs.)), the defending weapon special ability is, according to the rules as written, limited to swords, and has been so limited throughout the entirety of Dungeons and Dragons, Third Edition, including in the premium edition Dungeon Master's Guide (2013).

Curious Case #2: Spell storing

The most contentious of weapon special abilities, the weapon special ability spell storing says that

Any time the weapon strikes a creature and the creature takes damage from it, the weapon can immediately cast the spell on that creature as a free action if the wielder desires. (DMG 225)

Emphasis mine. Thus the DM must determine if ammunition and thrown weapons, after being loosed, are still wielded. You can read more about the controversy surrounding the weapon special ability spell storing in these threads from 2002, 2006, 2008, 2012, and also 2012.

The tables for randomly generating treasure don't prevent DMs from placing (rather than randomly generating) appropriate magic weapons nor do the tables prevent the PCs from creating appropriate magic weapons. For example, +1 keen crossbow bolts and +1 disruption sling bullets are equally legit, as are usually dumb weapons like the aforementioned +1 dancing longbow et al.

To avoid arguments, put magic weapon special abilities in the format used by the Magic Item Compendium

Prior to the Magic Item Compendium (Mar. 2007) magic weapon special abilities were not, as part of their typical descriptions, called out as limited to either ammunition, melee weapons, ranged weapons, or a combination of these. Authors and editors were relied upon to remember to have weapon special abilities apply to appropriate weapons. This sometimes led to omissions, mistakes, and strangeness. For example, the weapon special ability smoking (+1 bonus) (Lords of Darkness 180), as written, can be applied to any weapon but is potentially unbalancing and probably changes the DM's view of his setting significantly when every character who can afford one wields a +1 smoking crossbow bolt (166 gp 1 sp; 0.1 lbs.).

After the Magic Item Compendium, a weapon special ability's description includes the entry Property which specifies the type of item to which the weapon special ability can be applied (e.g. ammunition, melee weapon, weapon).³ (A similar history applies to armor and shield special abilities.) But, as Dungeons and Dragons 3.5 ceased publication only about a year after the Magic Item Compendium, this leaves the vast majority of weapon special abilities unspecified as to what they can be applied except in their description.

I suggest that if you've a particular weapon special ability you want to use dating from before the Magic Item Compendium you put it in a format similar to the Magic Item Compendium's and submit it to the DM for review. He'll appreciate you doing the extra work, and, after it's approved, you and he can really be on the same page as to how and on what it functions.

¹ The DMG implies that ammunition are weapons; if not, the only weapon special ability available for ammunition is brilliant energy (DMG 224).
² Random generation doesn't wholly prevent stupid weapons. For example, the DM can still roll a +5 brilliant energy returning net.
³ For comparison, Dungeonscape (Feb. 2007) contains the weapon special ability swarmstrike (+1 bonus) (Du 40), which is unclear as to whether swarmstrike ranged weapons bestow that special ability on their ammunition, implying they do but never stating so, but Drow of the Underdark (May 2007) has several magic weapon special abilities, all of which are clear as to what they can be applied and when a ranged weapon bestows the special ability on its ammunition.