Sorry if this is late, but I have trail ridden in my jumping saddle for years! With that being said, I have some joint issues that prevent me from riding any longer than 3 hours and I usually stick to about an hour and a half for my comfort (though I have gone up to 3 hours in this saddle), so I can't say for sure how well it would work for longer rides. I haven't had any issues, however, and have been very happy with it and am more comfortable trail riding in it than any of my Western saddles I was using before.
For what it's worth, mine is a Courbette Husar (very similar to the old Stübben Siegfrieds), so it has a deep seat, as well as knee and thigh blocks (though more minimal ones than many modern saddles), making it a very secure saddle. The first time I ever took it out on the trails, I kept forgetting I wasn't in my Western saddle and have had several Western riding family members and friends tell me the same thing when we switched horses on rides. I've also trail ridden in my Crosby Sovereign (the smooth bridle leather version of the Prix des Nations; pancake flat, no blocks or rolls, and very slick leather) and did alright, except nearly slid out of it when a horse struggled with his footing in the mud one day! It is not yet all the way broken in which may be part of it, and that was the only issue I had with trail riding in it, but my favorite is definitely the Courbette with the added security for trail riding. It's not so much that it interferes with your freedom of movement or your position, but is just enough to be helpful for when a horse slips, spooks, etc.