I can set the brightness of my laptop screen already via /proc. I can read and change the value.
Now I've set the value to 33 and then I went into gnome power management and enabled the option that it should reduce the backlight brightness being on battery.
That works, the screen gets darker.
If I now read out the current setting from /proc it still says it's 33. So I assume that there is another node in /proc to be used to control the brightness.
The node I use so far is:
I'm using the nouveau driver. With the nvida driver the brightness can not be controlled.