Now I have a rat with a great big hole in her neck instead of a great big lump. Huzzah.
(Really, it's an improvement, but wow is that a nasty-looking area right now. The vet had some wonderful things to say on the topic of enormous quantities of pus, and "I thought it was all out, but it just kept coming," and similar lovely dinner-table topics of conversation. I'm glad I wasn't there while it was happening, because abscesses smell really, really bad.)
It does not appear to be related to her previous tooth problems (that's an incisor, right at the front of her mouth, but it's abscessed before) or to any obvious injuries. She has developed the habit of hanging her head over the edge of the food bowl much of the time, so I expect that's a likely source of infection or initial irritation.
We have a rinse, and a cream, and more Baytril, the antibiotic which we rat owners more or less worship as a god. I am so not looking forward to trying to irrigate the underside of Niobe's neck. Rats do not like to be held upside down at all, especially in mid-air, especially over scary things like sinks. Perhaps we will be lucky, and she will have become too old and weak to struggle. Much.