Now I'm really curious too...
Since this couldn't have been a cheap guitar at the time, there has to be a decent chance that it was a fairly "established" guitarist. So maybe Google would reveal any semi-famous John Coles who were guitar players around the time period this guitar was built?
Which is actually still a bit of a question in my mind - I know the overall style is early- to mid-70s, but is that computer-style multi-pin connector to the pickups a bit more modern? Maybe 80s? Maybe I'm wrong, and these were available even in the mid-70s.
In any case, I did some Google searching for any guitarists named John Cole that might have been active in the 70s or 80s, and found a few:
There was a British rock band called The Movies in the mid- to late-70s with lead guitarist named John Cole. Not sure if he's still around; I can't find much info about this band. Example photos:
https://www.flickr.com/photos/khiltscher/albums/72157624754576866/with/4904910655Blues guitarist John Cole, passed away 2020. Seems less likely as the Joker doesn't scream "blues" to me, but who knows:
https://hsnorton.com/tribute/details/2994This guitarist John Cole was featured in a YouTube product demo by Monoprice, and sort of implied to be a "known" guitarist, but I can't find any more information about him. MIGHT be old enough to have commissioned the Joker?
Anyway, I searched for as many images of guitarists named John Cole from the 70s and 80s via Google Image search, hoping maybe a single image might show someone playing The Joker live, but unfortunately I didn't see anything.
The other possibility, of course, is that John Cole was just a private player who splurged on this custom guitar, and there is little to no information on the internet about him.