If I had a choice, I would hack a NES Advantage. It's already got rapid fire and slo-mo. Just needs some voodoo work to replace the original chip for NES with a different chip for PC Engine or TG-16 and rewire buttons accordingly.Maybe I'll see if I can find NES Advantage and do a writeup?
I have an used advantage coming in the mail. Once I figure it out I'll see about making a guide.