For instance, most people would find it less creepy if the harvesting was limited only to those who are already in prison, on long (20 year+) sentences; and it also seems that that policy would have less indirect harms.

There’s an incentive here to raise frequency and length of prison sentences, though. I think I actually saw a “death sentence for repeated jaywalking” scenario in some TV show, and IIRC it was actual caused by some medically-better-for-everyone “reason”, and it was pretty creepy.