I have an idea about this using the regular pepper's ghost set up in front of the window, but using 2 camcorders. The first to broadcast myself in a dark room( probably sheet a room of plastic or cardboard ), the second would be to see the trick or treaters so you could either comment on their costumes, or play on their reactions.I thought a small tv shouldn't give off too much light in the dark room. An overhead blacklight should give good lighting. Possibly even a pupeteered prop... Not sure how original the idea is though. If anyone has done this before I wouldn't mind hearing about the results.