Would it be possible (assuming some beneficial effect) to put blocks in the windows and hide them when they are in the camera view and return them when they are out of sight? So if the viewer is looking up at the second floor it is 'unblocked' but the first floor is blocked? Also can you maybe use a LOD type scheme with windows that are further away blocked, maybe with a texture image of the room rendered on the block to appear open until they get closer then remove the block? Anyway your stuff is always interesting.