I was just asking this question today elsewhere lol. I went directly to the organizers of a show series I want to go to. My old man is very uncomfortable in any type of bit I've tried on him (a jointed copper mouth snaffle being the least uncomfortable thus far). I don't know his history, but when he arrived his teeth were grown out way too long, and he has a lot of odd scarring on his tongue. I tried a bosal on him and he rode like a gentleman, so I decided to get a bitless bridle for local shows. The show personnel wrote me back saying it would be allowed because of medical reasons, but would be considered unconventional tack.