As of yesterday morning, Penwiper's recovery seemed to have plateaued. Better than Saturday, but still not back to normal. So off to the vet we went.
Fever 104 F. Normal for cats is about 101 F. The vet's comment was, "That explains why he's lethargic." Otherwise nothing obviously wrong: no swelling or anything like that. So they decided it was probably some sort of infection, took some blood, and prescribed an antibiotic.
Less than 36 hours later, he's almost completely back to normal. Appetite back, energy level and sociability much better. (Much to Oscar's chagrin, as he was enjoying the peace and quiet.) The vet says his blood chemistry is fine, but his white cell count was a bit depressed. Since that would tend to indicate a virus, which the antibiotic wouldn't fix, we still aren't sure what the problem was but are very happy that he's (almost) his old self again.
Whatever it is, it doesn't seem to be contagious. The Tiger Trio are all fine.