I doubt it would give everyone with some disability the right to board early. The argument would have to be that for a specific disability early boarding is a reasonable accommodation that is necessary to handle the person's disability. While I wouldn't rule out such a disability, my guess is an airline could say we can fully accommodate that disability by having you board last, along with needed assistance (and don't worry - we'll save some space for your comfort dog).

I think boarding people with disabilities, like boarding with children, grows from the assumption that they need more time to board and that letting them go early speeds the boarding process. Obviously there's a lot of room for debate on that point.