My favorite Mac shortcut is now option-command B, which I connected to an automator app which changes my external monitor brightness. It calls ddcctl which is a command line script which communicates over the displayport connection to control the monitor. This is a huge improvement over the annoying on screen monitor menu!
Thank you for that! The built in Mac display brightness controls only seem to work for the Apple Cinema Displays - they do nothing on my Asus monitor.
I just tried ddcctl ( https://github.com/kfix/ddcctl ) - and it worked perfectly over HDMI through a HDMI switch. I didn't even know what to look for - so this comment was a lot of help.