Well first of all its very a cool idea. I don't know how the keyboard backlight is wired, but it may be as simple as adding a splice from the little pulse LED on the front of the powerbook to the leads of the keyboard's LED, asuming they are accessable. try to add a small plug or connector (the common kind found throughout laptop wiring) so you can still disconnect the keyboard. I tend to think in hardware terms, I don't know if this is possible via software.
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PB G4 15" 1.2ghz - Pismo G3 400 mhz - iBook G3 500 "Jukebox"
"SawSmurf" G4 500 - Quicksilver G4 Dual 1ghz modding...