I think that alloted time is the best bet. It could be announced on the website/forums - but first you need a poll to see which time suits everyone best, which games people want to play, and who is actually interested etc..
When you submit your profile, there should be an option to add your timezone maybe? Days/Times you can and can't play?
Heh, this is getting way too complicated - but the #monkey-island IRC channel grew to popularity by using an allotted time....