I last did this about 11 months ago. I don't know if the rule is still valid since the DL merger. The SkyTeam mileage upgrade is different than the NW upgrade reward. I did use the CO upgrade reward to upgrade my cheap, bottom fare NW HNL-(asia) for I believe 20,000 miles each way, with no copay. I've actually done this 3x.

You should still be able to. The website says for Series 4:

Select Economy Fares Upgrade mileage required when upgrading from World Business Class to First Class on Northwest Airlines. Asia: Includes Asia, the Mid Pacific and the South Pacific.

Since there's a separate mileage withdrawl for full fare and select, it implies you don't need a full fare if you want to upgrade.