It ain't pretty, and it ain't ideal (you'd need to play around with the delay settings etc. to work depending on the speed of your machine), but you could alias "brighter" to something like:
Code:
osascript -e 'tell application "System Events" to repeat 50 times' -e 'key code 113' -e 'delay 0.1' -e 'end repeat'
and "darker" to:
Code:
osascript -e 'tell application "System Events" to repeat 50 times' -e 'key code 107' -e 'delay 0.1' -e 'end repeat'