[support] Non-unique account email addresses

This is something of a "if that won't work" condition to my last message (accounts w/no email address). If it is necessary for accounts to have email addresses, is there a way we could use an in-house blackhole type address for this? Assign the same address to any account which does not have a real address, and let the user login without needing an unique address to be assigned? I don't suppose it would be as simple as removing the key on the user.mail column? Even if so, if there is a module which would enable this to survive upgrades, that would be good. Luke