Rage Mechanics Question and Idea

There is a fair amount that would go into it, but technically possible. There are a lot of places that check for rage status because it interacts with battle a lot differently than other abilities. I think you'd need to start by not automatically putting a user with rage in the attack queue once ATB is full. Then you'd need to link it to an existing C1 routine for attack selection (maybe using the Control command with a bunch of branching code for rage)... I'm sure there'd be plenty more to it than that, but that's where you'd want to be looking to start.