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.
If the keyboard has media keys, then, generally speaking, F2 and F3 will be linked to brightness by default.