So I put her in the carrier and off we went to the emergency vet clinic around 9 PM. Lots of people there, so we had quite a wait, first for a room, then for a vet tech to take a history and vitals, then for the vet herself. Actually the wait was reassuring--the woman whose dog was taken straight back after she spoke to the receptionist clearly had an emergency, but Agatha's condition wasn't so critical. After the tech had come and gone, I picked up Agatha off the cold examining table and we sat together on a seat for 30 minutes or so waiting for the vet. The Clie with the scroll wheel for one-handed reading (Curse of Chalion), the other hand for petting the cat, and the wait wasn't bad.
Anyway, the upshot was bad constipation, apparently not uncommon with kidney disease where the cat is prone to get dehydrated. They did a lab test to be sure the kidneys were indeed functioning OK, then they gave her a stool softener, an enema, and expressed her anal glands while they were at it. We got home with an antibiotic for the next week (the anal glands showed some signs of infection) a little before midnight. She's finally starting to eat a little and seem more like herself, so I'm feeling that we got through this one.