With regard to online returns, that's what I understand as well. Again, the CRA can certainly answer that question.
By the way, I do my taxes old school, with a piece of paper. I fill it out and I send it away, so I'm not that familiar with how the electronic filing goes.
What I would like to envision there is that when you're online, there's a box there that asks, “Would you be interested in consenting to be an organ donor?”, and you click either “yes” or “no” there, and you send that off. It will go to the provinces' registries, and they would then pursue the legalities of getting you registered.