If Dao asks for enough in the way of damages, and tries to attach liability to UAL for his injuries, I would not be surprised at all to read that United argued in its legal briefs that Dao was a Rule 21 passenger. But as long as he is "reasonable," I figure United will go along with him being entitled to damages, be they Rule 25 or plain old "breach of contract" damages.
I think they’ll wind up paying him a lot just to make him STFU. They’re not going to worry about rules, other than the gag order in their settlement agreement. There’s no margin in going to court, takes too long, generates too much bad PR. Heck given how corporate life is it’ll probably be a different CEO by the time it hit court and the new CEO sure won’t want that trouble, pay him, make sure there’s timed payments so he’s insensitive to keep it zipped, and move on.