Heavy Gear Assault [Steam], an online mech-battle arena is now available in Steam Early Access. It has Linux support, but it has issues you should know about.
I don't have this particular issue, but there's a rather long forum post about it on Steam where lots of Linux gamers cannot even login in. I would keep an eye on that before you think about buying into it at this time.
An issue I do have, however, is that mouse movement for the camera appears partially broken. It seems you're supposed to be able to move the camera by just moving the mouse, but it doesn't do it. It seems to work at random. I pressed enter to chat and sent a message and then it worked, but as soon as I hit another button to do something in-game the mouse movement stopped again. Either the input in broken, or the keybindings are completely messed up. I don't seem to be able to adjust the keybindings either. I reported this a week ago and haven't received a reply yet.