I use game engine systems all the time for backgrounds. Its a good idea. Tribes is good for outdoor stuff like fields, rolling hills and deserts. Plus Tribes has a built in editor that lets you place building and such. the buildings aren't so hot but you can remove them, change the sky, fog etc for a good outdoor render. Not as good as bryce but great for a quicky render. I've also been collecting Unreal and Quake 3 mods just for their layouts and mazes for backgrounds.