I can’t imagine why this psak would be different for different people. When I go to the DMV and am asked to sign the back of my license, my situation is no different than anyone else’s.
And of course, if a relative was brain dead and the doctors were asking about pulling the plug, and about donating the organs, I would ask the shaila. Never hurts to ask a shaila. I’m not opposed to donating organs in theory- just in halacha.