Comment 5 for bug 344385

Scratch that, the display-brightness icons work, if you have them ALL (I initially tested with just the full and high figuring it would show notifications in just that zone at least, apparently not).

So for me a "sudo ln -s /usr/share/icons/Human/scalable/status/notification-display-brightness-*.svg /usr/share/notify-osd/icons/hicolor/scalable/status/" makes brightness notifications work.