Thanks!
The heat and humidity have admittedly been a problem for a few days. Between the first proper cold snap of the season and the heater running more often, the air in my flat's been a bit cooler and drier than normal since Monday. Running a humidifier hasn't helped all that much with the latter. Given that Kuzco was bright and chipper until her caught a case of the "zoomies" last night during our handling session -- seriously, he hasn't been so active outside of his enclosure for quite some time -- it may be that breathing in all that drier air is what's made him feel unwell.
For now, I've upped both his heat and the flat's heat, re-dampened his substrate, and put a new cover on part of the screen top to try to better keep some of that heat and humidity in. Hopefully all of that will help him feel a bit better! He was still alert and watching me as I did all of this, so he doesn't seem to be feeling too unwell.
In the event it's necessary, I was delighted to find that a new exotics-only/specialist vet clinic just opened up less than an hour away! Between that and the surprising number of ARAV-listed vets that are semi-local, I should be able to get Kuzco some proper help in the event he ends up needing it. I'll give him some time to see if he improves on his own, though. Hopefully this ends up being a case of catching something quick and ending it quick.