6 hours should be plenty, Assuming it takes you an hour to deplane and clear immigration (I'm being very conservative and giving you a pretty bad scenario), you can take the Airport train (which I've never done) to Dongzhimen (assuming 45mins). It's now 745. Grab a cab there to the Hyatt/Quanjude/Da Dong/Liqun (do an FT search for Duck). Assume 45 minutes (again bad scenario), its now 830. There is no reason that this meal cannot be eaten in an hour. Now its 930. Cab back to airport... 10-1030 PM..

Also for a midnight flight, since you're carrying bags and I assume will be checked through, there is no need for you to be at the airport before 11PM.

To put it short, you've got time. Get the duck.