The more I think about it the more I like my prescription approach.

There is literally no way a Gate Agent, Ticket Agent or Ops Agent can verify whether any letter from the Professional is authentic. It could be a forgery, it could be issued by someone who only claims to have the credentials, the Professional could have loaned their credentials to some third party, or the Professional's credentials could have been stolen. I'm also not sure how expansive the definition of the Professional is, perhaps that could be tightened somewhat to people who actually have a license in the United States to provide individual care.

If the prescription were run through a database first the Professionals's name and license number at least could be verified.