Hmm, I may not be conveying what I mean then. If the character currently has it...they keep it and stay playable.
If you want it for another...yeah, a repurchase might be necessary, or "This exists on this character, unlock for this account" could work, for things like ATs and power sets. IE...you have a Peacebringer, the account is flagging as Peacebringer being unlocked, etc.
I'm the first to admit that I simply don't know as much about the guts of the game as you, and you'd have a much better idea of what's possible.
There may not be any perfect solutions, but I'm thinking of what I've seen in other MMOs that worked, and was wildly popular with the player base, and generally got money forked over cheerfully rather the grudgingly.