I'm probably answering this myself by asking it, but if I wanted to make a plug-in for Override for Nova, I would just use/edit the Nova Files used for the Override TC... right?

Correct: you'd base it on those scenario files, just as you would base an EV Nova plug-in on the standard ones. The plug-in would of course be in the EV Nova format.

The real question would be to which add-ons directory you should upload it . . .