I have a Dell Latitude E6430 laptop, running Ubuntu 14.04, connected to a 27" Apple Cinema LED Display (which works fine). By tweaking the appledisplay kernel module a little bit, I can even get proper entries for the backlight in /sys/class/backlight and change the brightness using echo XXX > /sys/class/backlight/appledisplay/brightness.
As icing on the cake, I'd now like to have the brightness keys on the laptop (which do work for the laptop screen) also control the big display's brightness. Unfortunately, this doesn't work - I was assuming that as soon as a new backlight device is available, it's also connected to the brightness keys.
Any hints?