Organ donation panels are a sick Trolley problem come to life. I don't know either.

One easy way would be to make organ donation opt out instead of opt in. As in, if you do nothing, you're an organ donor if you end up brain-dead, and if you don't want to, you have to explicitly opt out. Alternatively, we could just say any brain-dead person may have their organs harvested, regardless of what they declared while still alive. After all, you don't need the organs anymore once you're brain-dead. (I'm specifying brain-dead, because if you're completely dead, then the organs are also useless)