My colleague Graeme Wood makes the case that we ought to harvest the organs of convicts slated for execution. There turn out to actually be two separate issues here. One, though we do sentence people to be executed, we never force them to give up their organs. But more interestingly, it turns out that those sentenced to death aren't even able to volunteer to serve as organ donors post-execution.
This last bit doesn't seem defensible to me. On the other hand, it's just very hard for me to know how to conduct moral reasoning about what is and isn't a permissible modification of a practice that should not be permitted. In other words, given that we shouldn't be executing prisoners at all, I don't really know how to respond to ideas about what things it is or isn't a good idea to do with their organs.