How do you plan on going about this clip-on surface? Will it be multitouch as well? The biggest problem I can see with your technique is that you'll need a different clip-on for every screen size; once you buy a bigger screen, it becomes useless. So, hopefully, it's not very expensive to manufacture; for example, I wouldn't go with capacitive testing due to this reason.
I thought about one possible solution but there's an arguably big disadvantage to it: users would need to wear a special glove. Each finger would have a tiny radio emitter which would need to be calibrated against the clip-on receiver. Even cheaper and perhaps more versatile would be to combine this varible-sized touch surface technique with a video projector that also enables variable-sized projection area.
