should prospective heart transplant recipients have to prove that they will do enough with their remaining life to benefit humanity, in order for the operation to be approved?

Distinguish personal morality from public enforcement. In a liberal society our personal purchases should (typically) not require anyone else's permission or "approval". But it still might be the case that it would be a better decision to choose the more selfless option, even if you have a right to be selfish. That seems just as true of traditional medical expenditures as it does of cryonics.