I’m going to answer “how do I combine Dervish with Lightning Maces,” rather than specifically how to get a slashing mace.
Aptitude Special Ability
The aptitude special ability from Tome of Battle can be applied to a weapon to cause feats that are specifically for another weapon to apply to the weapon with aptitude. It is likely that the designers meant just to let you switch Weapon Focus (dagger) to your aptitude longsword and similar, but the wording of the feat allows even feats where you never had a choice about the weapon type to apply to the aptitude weapon. This is frequently quite powerful, and occasionally completely nonsensical.
If you really want to use maces specifically, you could have an aptitude light mace, and then apply the Versatile Unarmed Strike feat to it. This is probably going to fall into the latter category for most groups, but it’s RAW-legal.
Alternatively, you could use an aptitude slashing weapon, ideally one with a large threat range (the kukri is almost certainly your best bet here: light slashing weapon with a large threat range), and then let Lightning Maces apply to it.
In either case, the Roundabout Kick feat works similarly to Lightning Maces, but for unarmed strikes: once again, aptitude can allow you to take the extra attack with your mace or kukri.
Combined with Disciple of Dispater, the kukris are looking at enormous threat ranges, which means you’ll score a critical on very-nearly every single attack that successfully hits. The maces are only somewhat smaller. Combined with Lightning Maces and Roundabout Kick, every critical triggers two attacks. Your number of attacks is thus more likely to increase rather than decrease; if you hit on the first two or three, you are statistically unlikely to stop attacking until the target is dead.
This is, of course, broken.
Were I your DM, I would allow you to have your slashing mace or allow Dervish to use non-slashing weapons without a second thought. On the other hand, Lightning Maces already is on my banlist. I also consider any combination of aptitude with a feat that couldn’t normally select the weapon in question to be something to be adjudicated by me, on a case-by-case basis.
For two-handed, reach weapons that you can use with Weapon Finesse, your options are basically spiked chain, and the drow scorpion chain (read: a slashing version of the spiked chain) from Races of Eberron.
Races of the Wild has several weapons that can be used with Finesse without being light, but none have reach. The lynxpaw is a double weapon (i.e. can be used as a two-handed weapon), and the elven courtblade just is a two-handed weapon. The elven thinblade is a one-handed weapon à la the rapier, i.e. it imposes restrictions on two-handing it.
Other rapier-like weapons include the ribbonweave from Races of Eberron and the quickblade rapier from Complete Adventurer.
The last such weapon I can find is the dragonsplit, from Monster Manual IV. These are one-handed but count as light for TWF and Finesse, and also have 19-20/×4 criticals. I’m not sure if they can be two-handed but since you’re not going for dual wielding I wouldn’t bother with them.
The spiked chain (or scorpion chain if you want slashing for some reason, or are a drow who can get easier proficiency) is far-and-away your best bet.
Alternatively, the feycraft template from Dungeon Master’s Guide II can make any one-handed weapon compatible with Weapon Finesse. It can also make light weapons use Dexterity instead of Strength without Weapon Finesse, which is often a bigger deal. I don’t know any one-handed weapons with reach, but the kusari-gama from Dungeon Master’s Guide is a light weapon with continuous reach à la spiked chain.
Best Answer
Now that the Advance Class Guide is out, the answer has changed (as @Lord_Gareth suggested it would in his January 2014 answer).
And now that the ACG errata is out, the answer has changed again. Changes incorporated below.
As of today, there are two published feats (three if you include Mythic) that expand the set of finesse weapons:
1. Dervish Dance
(which requires a 13 DEX, Weapon Finesse, 2 ranks in Dance, and proficiency in the scimitar) lets you count a scimitar as a finesse weapon (and add DEX instead of STR to damage)...as long as it's sized for you, used one-handed, and you aren't "carrying a weapon or shield in your off hand."
2. Slashing Grace
(which requires a 13 DEX, Weapon Finesse, and Weapon Focus) lets you use DEX (instead of STR) for damage with any one handed or light slashing weapon one has Weapon Focus in. As with Dervish Dance the weapon has to be sized for you. Additionally (since the errata)
Note though that it doesn't allow one to use DEX to hit with a one-handed slashing weapon unless one has at least 1 level in swashbuckler. Swashbuckler's get the Swashbuckler's Finesse ability which says:
That combined with this text from Slashing Grace:
lets swashbucklers (but not others with Weapon Finesse) use DEX to hit with a longsword or similar weapon. (A fighter with Weapon Finesse and Slashing Grace gets DEX to hit with her rapier but still uses STR for damage. If she switches to a longsword she gets DEX to damage but is back to using STR to hit.)
Also note (as per the original question) that Slashing Grace makes a bastard sword a finesse weapon for swashbucklers...but only while wielded one-handed.
Bucklers and Swordsmaster's Flair are OK!
The following FAQ was asked about Slashing Grace but applies to Dervish Dance as well.
Why No Love For Rapiers?
As an aside, Jason Bulmahn (Lead Designer) has said that DEX to damage was a late addition to Slashing Grace, and the design team didn't realize that it made a longsword significantly better for a swashbuckler than the more iconic rapier. To solve that, they've announced that the following feat will be released in the upcoming Pathfinder Player Companion: Advanced Class Guide Origins:
Mythic note: one more way to get DEX to Damage
While the core of this question is about adding to the set of finesse weapons, both Slashing Grace and Dervish Dance also add DEX to damage (arguably their bigger benefit). For completeness, I'll add one more DEX to damage option: Mythic Weapon Finesse.
If you happen to be playing with the optional Mythic rules, this is generally the best solution. The Mythic Weapon Finesse feat's only requirement is that you already have the general Weapon Finesse feat. It states,
So (unlike the options above, you can use any shield, flurry of blows, all of your natural attacks, etc.). It even tacks on the further benefit that > If you carry a shield, its armor check penalty doesn't apply to either the attack rolls or the damage rolls.