So I have a phantom power adapter that I want to attach to the camera, so I'm thinking of putting it on the tripod connector. Can I just glue a 1/4" 20 thread bolt to the top of it or something?

Well...you could. But you wouldn't be guaranteed that the adapter would be lined up with the camera. It would be better to do what Beachtek does and put a bolt all the way through the box, with a slot on the head to use a coin to attach it.