Oliver's hospital tank has cycled!
It probably isn't really necessary, though. I have done quite a bit of reading, and got some help from plantedtank members in diagnosing the issue. My best guess is that he had contracted clinostomum marginatum, a fish parasite known as yellow grub. Apparently it is really common among all freshwater fish species, lots of people get alarmed when they catch fish to eat and find them infested with the grub (harmless, easily killed when the fish is cooked). The parasite goes through its life cycle on three hosts- eggs hatch in the water, look for a snail, grow several stages in the snail, leave it and look for a fish host, can live up to four years inside the fish, finish their life cycle if a water bird eats the fish, leave the bird as eggs in the feces and hatch into the water again. It took me a while to find a diagram that showed the time period for each stage of the cycle- it lives up to four weeks in the snail and then must find a fish host within a few hours or dies. The only thing I can guess at is that snails introduced this into my tank- nerite snails are usually wild-caught and ramshorn snails are common carriers of the parasite too.
So if I keep Oliver out of his home tank for just over four weeks, pretty sure that whatever parasite lingers among his snail population will die out. I can't just get rid of the snails- it would be easy to pick out the nerites, but impossible to get rid of all the trumpet snails unless I tore down the tank. I have removed the few trumpet snails that were in hospital tank with him.
Now that I think about it, I remember seeing something similar to this parasite one one of my male cherry barb a while back. I didn't know what it was then, I gave the fish salt baths and the thing disappeared. None of the other fish in the main tank have shown symptoms, so I assume that if that was also yellow grub on the barb, it has all died out in that tank by now.