Assuming no checked bags then you could stay in international transit after returning from TPE and before departing to NRT on NW. If doing that then just make your way to the NW departure gate (security does not ask for boarding passes; if they do then show your itinerary) and check-in there. Make sure they understand your situation as they will question your lack of a Departure Card.

I've seen many passengers doing this at the UA departure gate after having recently arrived from India on TG flights.