The problem with that is it removes the incentive to sign up until you need an organ yourself. The vast majority of people don't need organs (at any given time

The problem with that is it removes the incentive to sign up until you need an organ yourself. The vast majority of people don't need organs (at any given time), so if you allow them to opt in only once they need an organ, very few additional people would register, basically defeating the purpose of the idea. I agree that it would be better than the current system though -- there would still be a small increase in the supply of organs, but not a big one.