I don't have advanced backlight installed as i am still tinkering with my xfce4. I will install it today when i get some time and try to locate the file for you.

EDIT: Ok i was thinking of advanced-power instead of the pre-installed advanced backlight :-) I think it can be easily implemented in a way that changing the brightness will write to to both files; screen brightness file and keyboard led one. Maybe be a different slider can be added to control the keyboard brightness. User rm_you maybe can shed more light on this.