Many, perhaps most, cues are made with bumpers held in place by a machine screw. It seems to me a bumper could be made such that the machine screw also holds a small threaded tube internal to the bumper... and that could be the anchor for the extension. At least for that type of bumper, would it not work?
The balance right 11" rear extension works like that. There's a replacement bumper with a hole in it, and the extension screws into the weight bolt on compatible cues. I've got one I keep on a Lucasi Hybrid. It works great, only thing I don't like is that it doesn't completely line up flush with the end of the butt.