Include entry in this under the organ and tissue donation programs with DMVs (i.e. partner with whoever is in charge of the "organ donor" part of driver's licenses)
Or automatically register people when they give blood.
Find an organization with GIANT reach but not necessarily an overlapping interest (DMV, United Way, a city government) and convince them to become a partner by making the default setting for inclusion into a bone marrow donor registry.
In germany, organ donation is the default for getting a licesne, and they have a 95% donation rate.
People obviously can opt-out, but make it easy enough to do and make it the default, and people will just go with it.