Is an addiction specialist the only type of dr who would've familiar enough with naloxone to try something off label with it?

No: Board certification indicates meeting specific requirements but doesn't guarantee that we know what we're doing! many docs w/o specific certificate have specialized skills. Worse, many boards are just junk, made up so that someone can achieve "board certified" status. Abms is only accepted overseer of boards (altho i suppose there are a few boards waiting in wings). Ask around for reputation.

Addiction Specialist: There are psychologists, psychiatrists, social workers, mental health counselors and certain physicians who may be experienced in the use of naltrexone. Most likely addiction programs in your area have personnel equipped to help you. My best.

Not necessarily: I think a big issue would be the condition that is to be treated. A specialist may or may not be willing to use Naloxone off-label depending on what research shows & the doctors familiarity with the off-label use in question. I am a psychiatrist and I am trained to use Naloxone for reversal of opioid OD. So, I would limit my use of Naloxone to FDA indications.