Backlight problem on PB Wallstreet II in Mac OS X

As said in previous post, there's a problem with backlight vs Mac OS X on Powerbook G3 Wallstreet. It seems to be a bug in video driver. The backlight can be triggered in open firmware :

backlight-on
backlight-off

after choosing pmu device.

I've put a (bad) video on YouTube that show the problem in Mac OS X on boot.