You would think that the "seamless" service between CO and NW, especially if you are on a CO-designated/NW-operated flight, would allow you to request/change a seat assignment on the codeshare portion.

This can be done at the E-ticket kiosks on the day of departure, but I would think you could do this online as well.

Does CO have an online support phone number? Give them a call. If anything, they would be able to access the NW res and change your seat.

The next time you're at an airport, you could ask a NW, CO or even a DL agent to go into the res and change the seat.