If McLain wants to ride a GP jumper in one because it works for him and the horse, far be it for me to criticize. I’m also pretty sure that McLain could handle riding with two reins if need be.
And that he doesn’t need one to get around the 2’6" local hunters.
The original question was about using them in the hunters. Whole 'nother thing.
I believe, and other posters have confirmed, that at A shows, with the exception of short stirrup, most judges would consider them unconventional tack and penalize the round.
At lower rated, local and unrated shows, I think it would just depend on what’s acceptable in your area and circuit. For instance, I know that in USEF hunters, Kimbwickes are considered unconventional tack, but they are very common and accepted in AQHA and 4H. Pelham converters may be acceptable in some local circuits, I just don’t know.
I personally don’t like them. I also am not fond of Kimberwickes, FWIW. Just my opinion and experience. That’s not a slam at McLain, or any other rider far, far above my level who finds them useful.
But comparing McLain to a teenager with a new horse who states she doesn’t want to cope with the second rein is a bit silly.