Kudos to Myrgard for spotting a post by Conner at Marius.net that states that Marius.net will soon expand to support Myth II as well as Myth TFL. Here's how it will work:

What is pretty [neat] (in my opinion) is that both TFL and SB players will be on the same server, in the same rooms, and can chat with one another. Also you will use the same login/password for both games. TFL players will only see TFL games, SB players will see only SB games.

Marius.net for Myth II is currently in testing and should be available soon . An official announcement is expected from Team Marius.net at that time.