Ookay, Hotkoffy. This has gone mostly beyond my stack of knowledge, and I'm relying on you pretty blindly.
I've corrected my DSDT, but that doesn't change anything. Now I'm using YOUR DSDT, and the problem still persists.
This is my 'ioreg | grep display' output:
| | | +-o NVDA,Display-A@0 <class IONDRVDevice, id 0x1000002d9, regi$
| | | | +-o display0 <class IODisplayConnect, id 0x10000039c, regi$
| | | | | +-o AppleBacklightDisplay <class AppleBacklightDisplay, $
| | | +-o NVDA,Display-B@1 <class IONDRVDevice, id 0x1000002da, regi$
| +-o IODisplayWrangler <class IODisplayWrangler, id 0x100000121, register$
Is the kext loaded? Is "Apple Backlight Display" what we're looking for? Uhh....
Also, I have an AppleBacklight.kext in /S/L/E. If I remove that, the Function keys (for brightness) stop showing anything at all.
Just to recap, my issue is this: The kext WORKS, but partially. It displays the little sun symbol when I press Fn+Brightness, but there are no white bars that increase or decrease. And the brightness slider in SysPrefs is always at max.