Nice idea but...
I have a late 2011 MacBook Pro with a ViewSonic external monitor.
I installed Brightness Slider, and at first it seemed great… Moving the slider up & down adjusted the brightness of the external monitor.
The problem however, is that it doesn’t maintain the setting, so every few minutes or so, the monitor would go to full brightness.
If a new version comes out, I’ll try it, but for now, I’m not using it.
mdbuckingham about
Brightness Slider