I recently booked the following flights on NW through the NW website:

SNA-IAH-DTW-CRW
CRW-ROA-DTW-IAH-SNA

I found out that 3 of the segments were codeshares with CO, so I called the OnePass Silver Elite line and was given "elite seating" on the CO flights. I was even given a CO RPN and told to check in at the CO ticket counter at SNA.

Now when the time comes for the flights, I plan to get up at midnight and call OnePass Elite to see about upgrades. And as JawsII has mentioned, if I don't get them at midnight then I will call them again before I leave from home and again at the airport.