I think a feature like that would be quite a pain for me to implement logistically. Lots of people are pretty enthusiastic about server favorites and originally I was pretty against the idea, but I will put some more thought into how I could do this.
Assumedly you would have it resolve a user's favourited server host BL_IDs into names and then you could simply compare those names to servers locally. If you wanted to have something like server tagging that would likely have to be handled between the server and the central RTB server, but I don't think it's necessary.
I mean not only would server tags likely be abused (morons checking every tag if it's a list or entering stuff like "AWSOME!!!" as tags if you give them a textbox) but really you should be able to look at the server yourself and see if it's the freebuild or RPG or whatever else that person hosts that you want. It's more than enough for the mod to make things easier. We don't need some nigh-self-aware AI to determine whether or not the builds on the server match a medieval theme to tell you if you want to join it or not.
Alternative thought: Badspot could (theoretically speaking) add a field to the master server output showing the host's BL_ID as well, thus finding server favourites would be really as simple as looping the servergroup to find relevant SOs and only displaying them.
