I replaced C402 (2 uF electrolytic) and the brightness drift is still there. It seems like the problem must be coming from the negative bias supplied by the horizontal board, which I can't get to without pulling the chassis.
I discovered that C402 was installed backwards all these years since I recapped the set!
You would think that would make it the obvious failure, but not so. With the somewhat cramped access and my tremors, the new install looks messy, but is solid.