Thinking about this more, I might be able to make sense of it.
I assumed that the plate-solving system would use my histogram-massaged image, which had a suitable dark background, and clearly displayed the Big Dipper and environs. But that's not true, is it? Instead, it uses my NON-histogram-massaged MASTER, which has a pronounced bright background (with chromatic ring), which of course would baffle any matching system.
If so, then I have to create a corresponding set of flats to further calibrate lights, and hopefully thus remove the bright and circular bias in my lights (even though those biases are NOT visible in Lightroom, and even though I did a heavy linear background flattening of this purely starfield imaging sequence).
(Then again, if you can show me another way with the darks and lights I linked to, I'd be very happy to know of it!)
EDIT: Astrometry.net had no problem solving my histogram-massaged TIFF:
http://nova.astrometry.net/user_images/2801584#original