Here is a link to some YouTube videos. The woman in these videos is disabled, and uses a power chair. Her dog, Laurel, is owner trained. Linda not only trained Laurel for service dog work, but she also competes in obedience, and dog agility. She started the Service Dog club that I belong to.

http://www.youtube.com/user/lkalberda#g/u

You can absolutely train a dog from a chair.

Linda has RSD, MS, and several other conditions. There are times when several days go by and all that she and Laurel do is stay in bed together. Other times, she feels better and they go eat out, run errands etc. Linda recently got a new puppy to train. Laurel is 5 and since it takes a couple years to train a service dog - especially for chair work for someone not as mobile as I am - she is starting him now. Most Goldens and Labs retire about age 8 or 9... and, Linda needs a dog for medical alert 24 hours a day. With two dogs, each one can have some down time while the other is working.