The backlit keyboard on the PBG4 15 and 17 use Fiber optics to direct the output light from two small LEDs into the keyboard itself. With the OD out of the way, there would be enough room to fit the LEDs it would just require a lot of finagling with the fiber optic wires.

The keyboard in the 15 and 17 is dropped into the topcase, below it's main level while the PBG4 12 is slightly above that point. You are probably going to need to dremel off the plastic and metal trim around the right side of the keyboard to make room for the FO wires. And there is plenty of room near the hard drive to rig up a tiny toggle for the backlight... No need to go through all the trouble of rigging up an ALS when manual is easier, and manageable.

And finally, if you aren't a heavy user of the FW 400 port, you could easily draw power from there. It's gotta be somewheres around 5-6 volts. I don't have a digital mm so thats an estimation. But it powered two 3.7v 2400mw LEDs with 1.5K resistors no problem. Break out the multimeter. I believe I used pins 1 (+) and 6(-) but don't quote me.