This gets me a little farther but not far enough :)  With my polkit
patch, which is under upstream discussion but probably will go in in
some different form, I can "pkexec ls", allowing me to "ls" as root if I
supply the root password.  With this patch, I should be able to run this
program as the "active" user:

   <defaults>
      <allow_any>no</allow_any>
      <allow_inactive>no</allow_inactive>
      <allow_active>yes</allow_active>
    </defaults>
   <annotate key="org.freedesktop.policykit.exec.path">/gnu/store/5l02a25w3gagrxf9nyfayayyiphfmk50-gnome-settings-daemon-3.18.2/libexec/gsd-backlight-helper</annotate>

But:

  $ pkexec /gnu/store/5l02a25w3gagrxf9nyfayayyiphfmk50-gnome-settings-daemon-3.18.2/libexec/gsd-backlight-helper --set-brightness 300
  Error executing command as another user: Not authorized

  This incident has been reported.

So not even an opportunity to enter a password.  Evidently polkit
doesn't see me as "active".  Probably an elogind thing.  Anyway, still,
going in the right way I think.

Andy