[sigh]
Both fish have been in the hospital/quarantine tank since Jan 23.
The female is in there precisely because this is so contagious; I don't want her looking fine and then discovering that she has the whatever it is and spreading it to my main tank.
I'm not putting her in her own tank because I've already got the main tank, a breeding tank for the endlers, a hospital/quarantine tank for the gouramis and another quarantine tank that is in use for another 2 weeks for some rainbows that I bought a week ago. I don't have any more room. She's just going to have to deal.
This certainly isn't a case of "wait to see".
Since it wasn't obvious at the outset what the heck it was (parasite, bacterial infection, ulcer or what) both fish are on an anti-parasitic medicated food (plus blanched veg as suggested), and I'm treating the tank with "Fungus Clear" which (according to the box) "treats a variety of fungus and bacteria-related conditions" and "Parasite Clear".
The treatment seems to be working and he's doing well so far. The big bump broke open & popped off some scales but the wound didn't go deeply at all and the swelling has gone down now. There's no sign of redness/infection, and the site seems to be healing.
I'm leaving both of them in the quarantine tank until I'm sure he's clear. He may been in there for years as he's got so many of the dark pellet-looking things festering in his body. But that's ok, it's a nice, decent sized tank for just the two of them with lots of real plants to play in. They can stay there as long as it takes.