No.
Unarmed Strike
The damage from an unarmed strike is considered weapon damage for the purposes of effects that give you a bonus on weapon damage rolls.
An unarmed strike is always considered a light weapon.
Improved Unarmed Strike
Benefit: You are considered to be armed even when unarmed —that is, you do not provoke attacks or opportunity from armed opponents when you attack them while unarmed. However, you still get an attack of opportunity against any opponent who makes an unarmed attack on you.
Unarmed Strike (Monk)
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.
Conclusion
In regards to anyone other than a monk, an unarmed strike is a light weapon that is neither natural nor manufactured.
Exclusively for the monk (and others with monk-like abilities), an unarmed strike is a manufactured light weapon, and a natural light weapon.
What is a Manufactured Weapon?
Any weapon that is not intrinsic to the creature, such as a sword, a rock you pick up, a club you make.
What is a Natural Weapon?
Natural weapons are weapons that are physically a part of a creature, such as a bite, a claw, a tail slap.
Unless specifically mentioned, (such as the Monk, creature listing or racial description, etc.) no one has a natural weapon right away. For example, no human has a claw attack, a bite attack, etc. without feats, spells, or other abilities that gives that to them.
Also, an unarmed strike does not cause lethal damage, provokes an attack of opportunity, and is a very unappealing option without Improved Unarmed Strike, and/or some other way to make it better. Monk's specifically mention that their unarmed strikes count as both manufactured and natural or intents and purposes - because almost no one else's do.
If you take Skip William's advice to heart (which is not RAW) then you will find him stating, "For purposes of weapon enhancements, an unarmed strike is considered a natural weapon," and "Natural weaponry deals lethal damage without recourse to a class feature or feat, such as Improved Unarmed Strike."
Constrict, Rake and Rend are not natural attacks. They are special abilities that, under certain circumstances, enhance natural attacks. Improved Natural Attack improves the damage of one of your natural attacks (per feat), not one of your special abilities.
Rake and Savage Grapple are pretty similar in effect. Arguably, they could stack, but Savage Grapple is essentially a better effect than Rake (or a better version).
It would be like taking Weapon Focus (Cleave). Cleave isn't a weapon (or an attack), it's a feat that can affect your attack with a weapon.
You could however, have Weapon Focus ([Grapple or Unarmed Strike or Ray]), but these are called out specifically as non-weapon attacks that can be "focused".
Some confusion comes in from the usage of the term "Special Attack" in the monster listings. They have "Special Attacks and Special Qualities"
Many creatures have unusual abilities. A monster entry breaks these abilities into special attacks and special qualities.
However, this is different than what is listed under the combat section as "Special Attacks", such as grapple or bull rush. Specifically, a Special Ability is either extraordinary (Ex), spell-like (Sp), or supernatural (Su). This is where you find Constrict and Rake. Rend is only found in the monster entries, like the troll, but it follows the same format and is listed as
Rend (Ex)
Signifying it is a Special Ability, specifically an Extraordinary Ability.
Best Answer
No, there are no such rules for sunder in particular
Excepting special cases for particular monsters, as with hydras or krakens.
The rules for sunder explicitly begin with
(emphasis mine)
There is an additional section for a carrier or worn object, but that still doesn’t apply to natural weapons.
In online discussions of sunder, the inability to use it on natural-weapon-using monsters is frequently brought up as a massive problem with the tactic (though far from the only one). I have never seen anyone mention any official rule for sundering them, nor have I ever seen such a rule myself, strongly supporting my broader claim that no such rule was published in a supplement. I specifically checked Rules Compendium, as the most likely location of such a thing, and it only reprints what core had to say (plus a little sidebar on how a DM should prepare encounters when a PC is using sunder).
As for houserule, I have not used, or seen used, any particular rules. Most seem willing to accept that sundering is just a dead rule, that is undesirable for PCs and (often) obnoxious and unfun for use by NPCs. There have been few, if any, attempts to rehabilitate it, and I’m not familiar with any. Your approach of using the hydra’s rules seems appropriate, though I wouldn’t guess that bone and hide, even fantastic bone and hide, has the same Hardness as steel.
There may be optional, variant rules for called shots, but....
I have to admit that I am not familiar with any official variant rules for called shots, but I suspect they exist (and homebrew versions certainly exist as well). However, I would caution against them: they are, in effect, like super-charged criticals. Many even run off of critical mechanics, but even when they don’t, by definition you are talking about something with a lower chance of a higher consequence.
The problem with this is that it degrades the stability of the system. The ability to predict consequences and prepare for them is diminished, and the game was already quite swingy. I would argue, then, that these effects are to the detriment of the game.
You may disagree; you might want something even swingier. That’s fine, as long as you have a group that’s on board with that, but I think it is important that you and your group all know the ramifications here: swinginess is inherently bad for the players. Mathematically, a dire consequence as a result is equally likely for PCs and NPCs (assuming they’re making similar numbers of attempts, which seems mostly reasonable), but while such bad luck against NPCs is much more likely than not to befall some random mook, the same bad luck against PCs will land on a PC 100% of the time by definition. And since there are relatively low risks of these consequences, it is more difficult to defend against them—player resources are already devoted to a number of things they need to defend against as it is, so it will be difficult to justify diverting some of them to protect against a low-risk event.
Called shots are far better than critical or fumble tables in that they are specific, planned maneuvers, so they avoid a lot of the huge narrative failings of critical or fumble tables. So that much is good. And since you are presumably giving up an attack or something else to attempt these called shots, your odds can be reasonable, and if the odds are reasonable, the effects need not be so dire. So a well-made system could mitigate some of the mathematical problems I mention above. But it is important that, whatever you choose, you go into it with eyes open, aware of potential problems. Unlike critical or fumble tables, they’re not insurmountable for a group that’s interested in this sort of thing, but they’re not something to add on a whim because it sounds cool. Consider it carefully.