How about a side opening, partial sleeve garment. The Patient would slip one arm through an opening with a very short, flaplike sleeve. There would be velcro patches to close the gown on the other side of the neck and under the sleeve flap (which would also velcro in place).
At the hospitals here they get everyone to put on two, the first like a, er, snuggie and the second like a robe. This way you only get exposed to calves and ankles.