@The Cubist - what if the mirror is on a plain object that's culled? You could put the camera behind the plain object on the culled side. also, if the mirror is on a wall, the walls could be culled plains too! thus if your camera is in front of the wall/mirror, you see the wall/mirror. If a camera is
behind the wall mirror facing back, it won't see the wall/mirror but just the rest of the "room."
I hope I make sense and this helps.