I wish I knew how to put video on the web. My son and I put a four foot/15 oz model rocket waaaaay over a quarter mile up. Made out of materials and prepackaged engines available at the local hobby shop. The high power rocketry hobbyists routinely put stuff up between 20,000-40'000 ft. Scary
Yes, it could be a problem if people could add a homing component and some ordinance. Not a very comforting thought at all...
Do the hobbyists play near airports?