It's definitely possible, but then you'd have to redistribute the modified version of the client yourself. So, you'd take the source, look for the background gfx (I think it's in /data), modify that with whatever image or texture you want, and then modify the menu code sothat it connects to your server instead of the lobby (this would be rather easy, its just changing the url that the code is linking to).
I'll look into it and give a more detailed walk-through if this method would work for you later.
Also, it would definitely be good if this functionality was possible without having to modify the client source/redistribute it, and I've talked to kripken about this kind of functionality, so it might be even easier to do in the future, just remember to pester us on the forums and irc