This was something that I remember talking about with some of the guys over on 360gamesaves.com when they were first starting out.
It is decidedly something they DIDN'T want to support. but it is possible.
Despite the possibility that you could do this, it's still more work then it's worth. Most games have explicit save point or check points. So even if the game was saved just before fighting the last boss you'd still have to play and beat that last boss. Not to mention you'd be missing all of the achievement up to that point. Unless of course you went through the tedious process of downloading an achievement specific game save, inserting your profile in there, loading it up to the memory unit, loading the game up and loading the save game. finishing the task required for the achievement, exit the game and repeat ad nauseam.
Personally I'd rather just play the damn game.