I have no solid information on this but I do know that NW has DTW-PEK rights that are currently used exclusively to fly cargo (NW could, and did in the past, fly passengers on this route but it seems cargo is more lucrative). A new PEK route would require (1.) China making it available and (2.) the US gov't approving NW's application over other competitors. Since NW recently lost the latest China route bid to UA (IAD-PEK was more compelling than DTW-PVG), they'd need to make a very good case for MSP-PEK.

I'm speculating, mind you, but I just don't see it happening.