The Steel Storm 2 game update video is embedded below, which was accompanied by a YouTube video description of "Quick update on what's going on with Steel Storm 2. While idTech 4 is different from DarkPlaces engine, the transition is going smoothly, although not as fast as I anticipated." So while they may be able to gain some greater features out of id Tech 4 over DarkPlaces, which is a heavily-modified version of the open-source id Tech 2 engine, the game's development isn't going too fast.
Steel Storm 2 was announced for Linux last year. This game sequel is described as "an old-school FPS in the spirit of Doom and Quake. It's spin off in the original Steel Storm universe. Linux is served first."
Steel Storm 2 is being developed by Kot-In-Action, which is the same studio behind Tomes of Mephistopheles, which isn't doing too well on Linux.