Just a brief update today. This past week we ordered two used NES controllers, and the first one came in today. We plan to cut the wire on the controller, discard the controller, strip the wires, and connect them straight to the Firefly node.
Another solution for this project could be to simply connect an NES controller to the NES, open the controller, and solder wires to the same traces that the NES buttons connect to. That way we wouldn't have to mimic the programming of the NES controller's onboard serial chip, and you could technically still play the NES with the controller if you wanted to.
However, since this is a design project, we will first attempt to create an interface to the NES that only consists of the plug and the Firefly node. If that fails, we might consider the alternative solution.