I wonder if people would be willing to actually do something like that? It seems like you could have a simple certification from a medical professional that you have a disability, as defined in the ADA. Rather like the handicapped parking pass... I know you can't just say, "I'm handicapped!", you have to have proof of it. So just provide proof like that to the state, and get a license of your disability. I don't think it would need to be any further regulated, too complicated. Just, if you have that license, you have a right to a SD.