Althought it's quite true the 360 just lacks the services through XNA for an MMO Server, what you could theoretically do (don't quote me on this though) is create a gamertag with gold account access that acts as your server machine and links to your online database on an always on Windows machine.
You could then program it to search the database (probably best to locally store on that rather than another proxy) and have that share the network data requested. The Live services is simply used to connect to machines, and retrieve gamertag information; past that the packet sending, sessions, etc... all that is in your control.
to be honest it's actually a sight easier than a manual system, and more stable than most automatic systems I've used in the past.