I have an external keyboard that's a common split-ergo design, probably one of the top 3 workhorses on amazon for "ergo keyboard" (Apple hardware is pretty, but it's not ergonomic).

By dumb luck I just found that it's ScrLk and Pause/Break that do brightness controls on my external monitor on os-x.

I wish apple made it as easy to remap these kinds of global shortcut keys as IDE's do.