You ever thought of doing backgrounds like unreal tournament. A seperate room with a box or some shape textured to look like sky, I had an idea of how to do it.
If you create this seperate room miles from your level. and its textured and stuff, make sure BACKDROP is OFF and basically before every sync have the camera filick to the position and then your players position always looking at the camera angle (DONT TOUCH THE ANGLE OR ELSE IT WUDN'T LOOK RIGHT) then sync, it would give the look off a proper 3d background, thus meaning the player could neva reach it. Oh corse if you player was standing in a completley boxed room they wouldnt beable to see the background. Has ne one eva had that idea?