My earlier question about adding human joint limits to a model has been partially and possibly totally solved.
Elbows only bend through a positive range (i.e. they don’t bend backwards).
Adding a Locked Track constraint to my Upper Arm (Left) that tracks to the Y axis with the Z axis locked seems to do this. It’s not totally human (unless they’re very flexible!) but it encompasses the human range of motion.
The model can put its left hand behind its back and everything moves as expected.
Thanks to everyone who chimed in and added their thoughts! The entire forum is such a helpful place to ask questions.