I feel like I'm on a roll. I just tried an older CyberBlade video driver mentioned in one of the other message boards and was pleasantly surprised to find out my backlight now turns off when power management kicks in.
The driver I'm using now is the Trident CyberBlade i7 AGP driver version 6.50.5482-10. Grab 5482-10.ZIP here:
Unzip the file, go to your advanced display settings in Win98 SE, and change your adapter driver to this one. I'm using "Plug & Play Monitor" as my monitor driver. Nice to be rid of that unsightly glow...