Come to think of it, the amount of one-shots I've seen where you don't have pre-generated characters (LARP or tabletop) is extremely low. I imagine this is because if a game has a specific tale that it is trying to play out within a small amount of time, it also wants the right characters for that game. Not to say that you can't run a one-shot LARP with player-written characters, I think it's just safer to have pre-written ones.

Campaign LARPs, on the other hand, are much more friendly towards player-written characters, but that's partly because the GMs/staff have much more time to work with the backstories they're given, to make sure those players will have a good time in the game.