"I have purchased many a NW ticket as a CO elite, at least on their website, you can't select the WP elite seating."

Not so. Both my wife and I are CO Plats who do most of our travel, both domestic and International on NW, primarily for the greater upgrade availability. In every case, I have been able to select a "P" seat on the chart at NWA.com. In those few cases where very few seats were available on a specific flight (such as only middle seats), when I made reservations, I did not select any seat and the NW computer always put me in a "P" seat automatically.

Are you sure your CO number is entered into the NW reservation correctly and your name, etc. and everything matches? You really should be able to select "P" seats. The other day, at the 90 day point prior to JFK-NRT-PVG, I went to the site as selected seats 73AB, about as "P" as you can get, with no problems.

Note that as an aside, the seating charts actually open between 9 and 11 PM Eastern Time the night before the 90 day point, if you really want particular seats