Easy compromise: if you die without a seatbelt (or helmet for a motorcycle) you are considered to have fully donated your remains for medical and scientific use, no opt-out or exceptions.
Dying from a crash doesn't mean you don't put pressure on social services. You could die at the scene (first responders and paramedics still), on the way to the hospital, upon arrival, or hours or even days after.
This does nothing to alleviate those pressures and the number of organs that are useful for transplant after a violent crash (that kills the occupant!) is basically zero.
