Have you tried the Serious Sam Second Episode demo? SS has a "cooperative" online mode, where two or more players work as a team through a level. That could be fun. Later.
The interest on this thread is only trickling in, but I do hope to act on it. We only need a few for fun to be had... at least set up a room sometime to try it out...
If it worked out, some of the team games in UT (like Capture the flag and Assault) would be more fun if there were actual teams that would work together. Haven't tried the other games you mentioned...