I believe the backlight will stay fixed at 100% until someone figures out
what PCI register to poke (or reverse engineers the original firmware hard
enough to find this detail.)
In the meantime, the screen can be "fake-dimmed" using xrandr, for example:
xrandr --output LVDS1 --gamma 9:9:9 --brightness .5
I say "fake" because this will only make all pixels a bit darker in color, it
will not reduce the backlight. But it will help make your eyes strain less at
night, which is better than nothing. The gamma setting is a way to try and
put some contrast back in.
It can be undone with:
xrandr --output LVDS1 --gamma 1:1:1 --brightness 1
Hope this helps.
