I'm confused about where the prohibition is. N991SS is an example of an FAA aircraft registration number for a balloon; I looked it up myself on the FAA N-number inquiry site to confirm. Your link does not talk about balloons at all. Where does the FAA prohibit weather balloons here? It sounds like it's just physically not possible to carry a transponder or reflector on a small weather balloon? But that's different than what the earlier posts were suggesting. It seems like it's physics preventing ADS-B on small balloons, not the FAA. If you did manage to get ADS-B on your balloon somehow, could you not register the balloon like the owners of N991SS did?