Scratch that, I fixed it.
I couldn't resist trying to figure out what the heck was wrong. It was clear that something was just being over driven in the PWM circuit that controls the backlight of the screen. so I took it apart and went on a hunt.
VR2 was cranked way, way up (part of the PWM circuit) so I dropped it down to around 1.25v (from 3!) and all is well now. No more whine, no loss in brightness. I'll tool around for a few days and see if this holds up. It worked on both T9 units I have in my house now, so fingers crossed.