I think the hardest part is gonna be coming up with parts. You'd need an expansion port plug, NEC proprietary, so you can only get it by removing it from a turbo booster or cd-rom attachment. Then you'd need a card slot, which you could get from a dead PC Engine or TG... but by the time you've tracked all that down, why not just use a kisado or do a permanent switch mod?
I guess it'd definitely be a cool novelty. ;-)