I’m doing it in UDK. I really wanted to make it Linux compatible but there ended up being no way I could really do it given the tools at hand.
Doom3 source code is a mess at the moment and ET:Xreal is non-functioning or dead. Also, UDK had the documentation that I needed.
A big problem with Linux right now is the state of modern distributions. If you’ve ever tried gaming with compiz/mutter/unity on it kills your performance. Plus Pulseaudio sucks. It really is becoming a bad platform for games.
The problem with Xfire any more is that nobody else uses it. I have 200+ xfire friends and I maybe get 15 on at a time. Everyone’s moved to Steam. So, yeah it will definitely work with Steam but I don’t see why it would be mandatory as it’s a free game.

