Apparently it really is a UV tattoo, at least insofar as it's a deliberate, permanent mark, with discernible meaning, made by injecting some sort of UV-reactive ink under the skin. Certainly not a tattoo in the sense of a tattoo being a piece of art that one chooses to receive from a tattoo artist, though, heh.
Funny you should suggest the dermatologist, since this whole thing started with her seeing a faint scar on the back of my neck. Interestingly, it didn't fluoresce under the fancy dermatological black-light, but it does become visible under a cheap "party" black-light. Haven't really thought about removal ... at this point it's more of an interesting puzzle that I'm just trying to figure out.
The dermatologist was actually pretty clueless. It was only through my own online searching that I learned there was such a thing as a UV tattoo (which is why I thought it might make sense to ask here). I've been talking with the artist who did my tattoo about ten years ago -- the [non-UV] tattoo I chose to get, I mean, lol -- and she's been great: really respectful and helpful. She's checking with one of her connections in the medical field to see if they have any ideas about the stuff that she couldn't explain herself. I'm also in the process of contacting some other folks who were doing tattoos in the mid-80s and who might have thoughts about how someone might've done the UV thing before it was officially a thing. If neither of these pan out, then, yeah, my next step will be trying to reach out to someone who specializes in tattoo removal -- I imagine they see all kinds of crazy stuff, and might have some theories.