Seems that @BilliardsBall is having trouble distinguishing the morally praiseworthy from the morally obligatory.I'm not sure. I may do.
But whether or not an individual WOULD do such a thing is not really relevant. We may both agree that cutting off your own hand to save another person's life is a very nice thing to do, much like running into a burning building to save someone from a fire, or donating an organ to save someone dying from illness. These are certainly brave and noble things to do.
But that these things are good things to do is not the position you are arguing for. What you have to argue in favour of is FORCING SOMEONE BY LAW into cutting off their own hand in order to save the life of another. You have to argue not only that it is morally good for someone to run into a burning building to save a life, but that they should be PENALIZED BY LAW for NOT running into a building. You are arguing not that simply donating an organ is a good thing, you are arguing that it is morally justified to FORCE PEOPLE to donate organs against their will.
Would you agree with that?
Supererogation - Wikipedia