Quoted from CaptainNeo:To expand on your idea. What if you had a padded bar, but the bar stayed stationary. And you put a flipper button on the bar where your thumb would hit. Have it mechanically or electronically move a device on the end of the bar to push the button in. That way the bar would be stable, not sliding back and forth. Would give the person duel control and both would feel like flipper buttons and respond as such. You would only need the pusher to move 1/4"in and out.
I have been thinking of similar devices for my wife (quadriplegic). I think it is definitely doable, just need to find the time to work out the details and build.