If the horse won't canter except to a fence or on the trail - well then it's probably not going to like dressage very much!!
I do like cross training (ie jumping, trail riding) for keeping a dressage horse fresh, thinking, and to help with minor issues (ie I used fences to help my horse relax about the changes - he got very uptight and high strung about them, through a course of jumps he was more focused on the jumps than freaking out about the changes), but if the horse wont canter in the first place, well you can't set up jumps in your dressage test... I'd pass.
They say money doesn't buy happiness -- well happiness doesn't buy horses!