Dim screen brightness of MBP, using AppleScript (and while using a secondary monitor)?
Having a secondary monitor was not the issue, it was that I also needed an external keyboard.

When using an external keyboard, the brightness key codes change from 107 and 113 to 145 and 144 respectively, to lower/raise the brightness level.

For anyone interested, the correct key code events that works with an external keyboard (tested/works with an external Apple keyboard) are:

To lower the brightness all the way use:

tell application "System Events"
    repeat 16 times
        key code 145
    end repeat
end tell


The above code will lower the brightness all the way, regardless of where it's currently set. If it's already set at anything below 100%, any extra lower key code events, while still executed, are done so harmlessly and are just ignored when the above code runs.

To raise the brightness to, e.g. 75%, use:

tell application "System Events"
    repeat 12 times
        key code 144
    end repeat
end tell


Note that if you want a smother transition, add a delay 0.02 command to each repeat loop, otherwise the transition can appear quite abrupt.