Competitive trail challenges have nothing to do with conformation, but how well he listens, works off your legs and is a willing partner.
OK, we have two different competitions here.
A) Trail Competition = riding over obstacles, opening gates, backing over things, etc.
B) Compeditive trail = 25 or 50 miles over varried terrain. This is a timed event, you must not drastically exceed or come in under the alotted time. The goal is to have your horse come back from the ride in as close to the same condition he started in as possible.
I think the op was talking about b. The biggest thing with compeditive trail is conditioning. I have competed on ottb's, an appy and an arab, and my friends have ridden qh's and paints.
The biggest thing with a muscular stock breed is that they don't cool out as quickly as the lighter breeds. At the vet stops your horses pulse and respiration has to come down, which takes a little longer. Also many qh's don't have as much endurance. That said I've seen many successfuly compete at rides, so go for it, and have fun :)