To explain further about why this is not possible, World of Warcraft can unfortunately only retreive information from inside of World of Warcraft. It also can not update an AddOn when a file is changed (It only loads AddOn files when you (re)load the UI)
But you might be able to run World of Warcraft in maximized window mode, having something like this installed:
http://www.math.cmu.edu/~ploh/battery.shtml
or
http://mattcollinge.wordpress.com/so...er-meter-plus/