Perhaps consulting someone in the field of sports medicine could help. I've seen some amazing things televised that allow those less fortunate to do just about everything under the sun. Sometimes better than those without physical limitation. My guess is that a specialized physician could adapt an apendage to better suit the needs to the particular person.

I know a guy who made something like this but I do not know if he sells it yet. He may make one for you even if he is not selling them yet. Here is the website.

http://www.innovativetreestands.com/about.html

I seen an article a year or so ago, and he made a fiberglass part that was attached to the bow and his kid just stuck his stub into it at an upward angle so that it wouldn't fall after the shot.

I wish I could remember atleast the name of the magazine...but I don't

Maybe if you know someone that does some fiberglass work, you might be able to come up with something.