The therapeutic riding program that I used to volunteer for had one miniature, but they did not use him for riding. He was handy for things like showing how to groom or measure a horse.
I am currently involved in a program that uses many of the horses that have retired from that center (see Angel Horses in my signature below). Because the horses are retired, there is little or no riding, so most activities would be appropriate for minis. One program they offfer is an after school program for middle school students that have little or no experience with horses. I have taken two of our minis to that program for the students to lead over a trail course and have also given them all cart rides. There are photos on our website (www.ontargetminiatures.com) in the gallery. At least one student went from no horse experience at all to now wanting her own horse some day.