Hi Sonicrobby. Yes, this is quite easy with our good old friend "Activator".
By default, Activator doesn't ship with a brightness action, however there is a free one in Cydia called "Brightness Activator Action". Download and install this.
Then configure "Activator" in Settings for "Anywhere" and set the Power "Connected" and "Disconnected" actions to "Brightness Up" and "Brightness Down" respectively. Ignore the fact that the text says "by 10%".
You then have another setting in your main iPad Settings panel titled "Brighness Activator Action". This is the actual value that the brighness changes by each time the action is triggered.
So, from your request, if you set the slider bar to about 60%, and start your brightness at 20% when unplugged, it should jump to 80% (20+60) when plugged in and drop again to 20% (80-60) when unplugged. As I say, you may need to fiddle with the slider a bit to get the setting you like.
Hope that helps!