There are problems with that:
#1 - It would require a centralized, master server for everybody to meet up at. SRB2 doesn't have this, and probably won't have one.
#2 - Tying into #1, without a "master server", the only remaining reason for a lobby would be to stand around and talk -- you could simply make a map with no rings in it if you wanted to do that.
#3 - I can't speak for SRB2's netcode because my router prevents me from playing online games like that, and I never checked it out on 56k, but anything over six people would probably cause massive lag. If SRB2 had the master server to fuel the lobby idea, I assume it would be pretty popular, meaning it might quickly overload.
A standard listing of active servers would probably do just fine.