Exporting scoring data would work something like the "Now Playing" data in this thread
. Stepmania would write to a file and something else would have to read that file and pass it to your special device.
On the Stepmania side, you'd have to create an actor on ScreenGameplay that listens for JudgmentMessageCommand, and uses that and the lua interface to figure out what data to write to the file. I can write up some example code in a couple hours.