Actually, you CAN, you just have to be willing to look through a lot of code, and know Python.

It's written in Python, so with root access and a code editor, you can tweak it, since it doesn't need compiling. I presume it's because it starts on the assumption that the charger is in some state (whether null or charging, apparently not unplugged, though), and then the first execution checks for charging status, finds it on discharge, and then defaults to normal if-charger-state-is-changed-to-not-charging-play-this-popup-notification. Should be pretty easy to make it assume it's NOT charging upon load, since people probably don't care about getting a 'charging' notification upon boot if they ARE charging.