Not quite appreciative of the scenario as you describe it, but we could work a similar design out quite well; As an example:
"Improved Weapon Finesse: [Feat, req: Weapon Finesse, Dexterity: 21, BAB: 6] or [Enhancement: Ninja Spy/Thief-Acrobat: Req: Weapon Finesse]: Benefit: All finessable weapons (Or short sword/staff, for respective enhancements) and thrown weapons now use dexterity for damage."
Then, add in a tier 5 innate for Shadow Dancer that allows finessing of additional weapons. That'll give it the unique element, while still preserving the potency and early level use of finesse.