I know the Neo Geo controller extension wire works for Famicom controllers, but would a Neo Geo controller ITSELF work on my Twin Famicom? I'm okay either way, just curious is all.
Nope, unfortunately not.
Nope - you can't just plug it in. But the Neo Geo controller is just a bunch of switches. It would be trivial to build an adapter board with a shift register and some resistors.
-Ian