Walkers, I'd suggest to look for the riding program for disabled in your area, go there and talk directly to the trainer to see what they use/recommend. I've never heard about a special saddle for handicapped people (not saying they are not out there, just saying I've never seen one), so may be you start with the normal saddle and then add extras on top of it.
BTW, I talked to the lady with disabilities (she had a stroke so was liimited in her movements) at the last Expo, and she was looking for very deep A/P or dressage saddle at that time.
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