This time the vet spotted actual signs of an allergic reaction (which she didn't show before that I recall) both where she's removing hair and under her chin. He gave her a shot of a steroid which should easy the allergic reaction for 3-5 weeks, hopefully past the time that the allergen is present. And also, hopefully, breaking her cycle of over-grooming.
In penance for taking her to the vet, I was sentenced to providing a lap for a long afternoon's worth of cat naps, and to provide head scritches on demand. She demands lots of head scritches. It was a nice way to spend a lazy Columbus Day holiday, actually.