Thanks Channa. I think I'll just do the stopover in SEA. The flight from NRT gets in at 8 in the morning. There is a flight that leaves the next day at 1 a.m. and it works as a stopover for M class availability purposes, but is within 24 hours so doesn't count as a stopover for pricing purposes! With this fare stopovers are allowed, but cost $100 each.
By the way, I noticed that on many days there is M class transpacific to/from SEA, but no other US location. NW must be having problems filling the SEA flights.