Cool beans! I've wanted to do something like this for a while, but I see one major drawback:
My overhead light is controlled through a wall switch, meaning the relay would have to be embedded in the wall behind the switch (piggybacked as you called it in the video). The problem this leaves is that I must have wires coming out of the switch somehow in order to control the relay from external sensors!
How did you manage to do that without an ugly solution (no faceplate, etc.)?