Well, I -don't- use such a mechanic, but you bringing it up conjures multiple possibilities instantly to mind which could be very cool. The simplest being that having a mortal's true name means that when casting spells on that person, they get no saving throw, even if they normally would. If that's too strong, make it so that they don't get will saves, or that their saves are merely penalized by some ungodly amount (that COULD be bypassed, but only with a LOT of buffing!)
Since saving throws are so useful throughout the game, being deprived of them would be DEVESTATING. However, another option would be that the possesion of the True Name means that said person is never "out of range" for any spell. In other words, that the caster possessing someone else's true name can modify any spell's target and range to: Range-Infinite, Target

erson'sTrueName. This has some interesting thoughts to it, because it brings up the nature of trust and whatnot... if having the name merely means no saving throws, there's no reason you'd EVER give it away to ANYONE. But if it means being able to be effected even at distance, you MIGHT give it to a close (spellcasting) friend or companion, and then have to trust that he/she'll never betray that trust, and never give that name to anyone with less noble intentions.
Or you could treat it like having an outsider's true name, in that you can summon them and have them do your bidding. Would fit the current rules fairly well, I think. Perhaps make them suseptable to Charm and Teleport spells only, that sort of thing.
Thoughts?