Ok, so you are saying that if someone were to retrain a horse from to trail barn to be a good riding horse, what steps would they follow, right? First of all, not moving off leg with a horse like this is actually one of your smaller problems. Horses like this (like marecare said) aren't only dead to legs, they are dead to the world. They find some kind of sick comfort in their routine and are terribly insecure when offered something new, which is why they tend to be very buddy sour. Their comfort isn't in themselves, its in their job, even though it isn't a good job and they are not usually happy with their jobs, they are content with the fact that they know what is expected of them. I have found that many of these horses are so stuck in their patterns that they are extremely skeptical even when someone offers them a better deal.
I say this because in order for the horse to learn efficiently, they have to be in the correct frame of mind. Horses on a trail string tend to be extremely introverted and to immediately start correcting the horse is going to send them into defensive behavior, not learning behavior. I know for a fact that people can "beat them through it" and teach them to move forward to avoid pain, however I do not do it or recommend it. I do use increased pressure to get a response and immediate release to reinforce, but it is always used with an understanding of the horses frame of mind. If the horse is ready to learn, then they will retain and refine what is being offered. If the horse is not in the right frame of mind, then they will react out of fear or avoidance. Both can get the same physical result, but the first horse will start asking "what can I do for you next?" while the second one is saying "what do I need to do for you to leave me alone?" It is sad how many people mistake "obedience" for "correctness" and "willingness".
To get the horse in a mindset where they are ready to learn, it is best to start making them think. Allowing them to just be a horse for a while works wonders, regular handling without too many expectations but still requiring basic manners gives a horse stability and solid leadership. To be patient with the horse shows the horse the ability of the human to be the leader and to show them a better way. Once you have the mind, they are willing to work with you on their body. Then, once they realize to move off your pressure instead of into it, it is relatively easy to teach the horse forward off leg. Remember, trail horses have been taught not to listen because the people that are riding them don't know the language. They can be retaught, but not in the environment that they are in.