Have you solved or worked around your Debian Bug#688828: xserver-xorg-video-radeon: brightness controls don't work?
If so, how, please? I have precisely the same problem you have, and can confirm similar symptoms in detail, including the unresponsiveness in /sys/class/backlight. My machine is no Mac, but its video chip is an AMD 7570M (despite its 7xxxM model number, apparently really a 6xxxM, Northern Islands series, Turks architecture, similar to yours).
If you have a solution or workaround, please advise. Thanks.