This is an excellent question. I saw one Youtube video where the autoharp player had a pad on the side of the harp for his arm. It looked like a pad used for the chin on a violin. This prevented the corner of the wood from digging into the arm and cutting off blood supply. If you look at pictures of Brian Bowers playing, often he holds the harp up very high so that he is strumming by his ear. This allows the left hand better access at the chord buttons. There is one more answer to this, but it is expensive. Hire a luthier to build you one of these:

http://www.andymanson.co.uk/img/Autoharp.jpg

It is a chordbar assembly with cantilevers which let the hand press it from one position and push the bar at another position. The luthier who built this one said it was for a client with palsy whose hands were not strong or limber. Good luck.