CaptainHellrazor:
As far as I am concerned I put in as much time into character generation as the players do, if they quit the game then they have zero rights to how that character is used from that point onwards.
As a polite bit of advice (and you may already be doing this already), but it's probably not a bad idea to make that very clear at the start of your games that this is the case if it's a policy you're going to pursue, because I don't think that's the default understanding which players have when they enter into a game and it's not the principle upon which intellectual property works as a default.
It's not to say that some players aren't totally fine with the concept and indeed that there's a certain degree of expectation on players to manage their exits with the remaining players and GMs in mind, but I know a fair number of players who would seriously object both to the idea that a GM automatically puts in as much work or has as much ownership of a character as they do, and if the GM decided to continue to use their character after they left a game (regardless of the circumstances) and they had no say in the matter or right of refusal.
To address the original point, if you want to bring back ex PCs in a game at the finale, then I'd only do so if I'd either got their express permission, or at the very least that I'd been very explicit that this was going to be the case for players leaving the game from the beginning. IMHO, better practice would just be to use existing NPCs that you had written in this role.
As for whether players would be interested...I dunno. Personally, it wouldn't appeal to the bits which make roleplaying good for me - to pick up a pre-built character for a short period of time and without much to get invested in - the effort compared to what I'd get out of it isn't for me, but other people may feel differently.