In my limited experience, trail riders want to spend the day walking along on a quiet horse as they enjoy the conversation and company of their trail riding companion. Dressage riders school in the arena with an occasional hack around the barn. There doesn't seem to be a lot of cross-over.
Endurance horses lead a much different life than either the average trail or arena horse. They're legged up to cover 25 to 100 miles in a single day and are often ready to repeat the same thing the next day. Most endurance horses are awesome trail horses: they have no fear on the trail, but they often lack some fundamental training in proper self-carriage. That's where dressage on the trail comes in!
I had a great trail ride today with Speedy G, my once endurance horse turned dressage horse in training. We worked on lengthening our walk on the way out since he was POKING along with no desire to walk past the lush green grass growing along the trail. On the way home, we focused on containing his "racey" and strung out walk by working in a more medium, march-like walk. During the middle of the ride we worked on gently changing the bend at every ... well, bend! I also used the bends in the trail to work on posting to the opposite diagonal, which can get tricky on really bendy trail!
Trail rides are no longer just about conditioning Speedy's cardiovascular system and strengthening his soft tissue. We now use our time on the trail to focus on the skills that we're learning during our dressage lessons. Too much arena work gets boring for Speedy which has lead to some pretty cranky and non-productive behavior. Working on the trail has kept his mind fresh while keeping his body conditioned at the same time.
See you on the trail!