To completely disassemble a driver is a fairly time consuming activity, at least the way I was doing it with FDisasm. This isn't really something I can bang out real quick, so probably won't be getting to it. Maybe someone else can.
The backlight is actually controlled by the M50753 PMU microcontroller. I'll see if I can find the backlight commands and see what exactly controls it, might be an on-board DAC or could be just a GPIO. The MacOS backlight driver just sends commands to the PMU.
My PMU dump from a non-backlit Portable (which I believe Rob did for me) exactly binary matches the dump from a PowerBook 100 (which is obviously backlit). My guess is the backlight was jettisoned from the Portable late as a cost-saving measure and then they put it back after consumer complaints.