Excellent call:
http://webdiplomacy.net/board.php?
gameID=65923is the new game url. It is anonymous and without in game messaging. That would mean that the only way players would be able to establish outside communications would be to mail their email address.
If any person receives a message containing that, they can photocopy it and send it to others to let them know about the cheater.
As far as the time limit, 10 is the max that can be set. From what I have seen on the older news groups, 2-3 weeks was typical, but that was also factoring in sending and receiving messages from the GM, so this should work fine.
Postal mail is actually pretty quick and will arrive in the next day if you time it just right. Even with 2-3 day turn around, you can get 3 messages back-and-forth before the deadline.