In conjunction with shift pbbuttonsd should set the brightness to maximum level. That works fine on the iBook G3 but doesn't work on the PowerBook G4 (PowerBook5,8).
And this is a Gentoo-specific bug exactly why?
It might be a limitation of the keyboard. Can you try on your G3 to see if the shift + fn + F2 has a different keycode than just fn + F2? If not, I guess perhaps report this to the upstream developer.