Tis an option, tho standbys wont have much fun waiting to see if someone quits... Probly just go w the normal rules, if you sign up, you're signing up to play, and if you quit 2 hours into a game you knew was 6 hours, then you get tar & feathered, strung up by your toes, and beaten with lint-covered dust mops. I think that's our policy.
Maybe if you have a list of extra people for the game, they could be standbys to take over for those leaving. Not the best solution, but could be useful.
Clagr