Comment # 51 on bug 1036463 from Martin Wilck

(In reply to Marcus Meissner from comment #50)
> FWIW libgphoto2 uses the ruleset to hand out user-access-permissions for sgX
> devices for a limited set of cameras and picture frames that it can control
> (most cameras are just libusb users and do not use sgX). It assume sg loaded
> for this I think.

Yes, that makes sense. The rules that I'm seeing serve only the purpose of
setting 'ENV{ID_GPHOTO2}=1' and 'ENV{GPHOTO2_DRIVER}="proprietary"'. 
GPHOTO2_DRIVER doesn't seem to be used anywhere, but ID_GPHOTO2 is used to set
the uaccess tag, as you're saying, and COLORD_DEVICE if colord is installed.

Anyway, comment 5 should be fully sufficient to make this work.