I don't know anything about the dining plan, but we do have the DDE card. We ate at Brown Derby and got the Famtasmic Dinner Package..a fixed price for an app, entree and dessert. They do not take the DDE card for that. BUT...I knew DH and I were going to have a few cocktails before dinner, so I asked the waiter if he would ring our drinks up on a separate bill, and he said, "Of course, no problem."
Just another tip...if you like mixed drinks, and are planning on having more than one, request a "double." My DH and I did this all over WDW (well, in Marrakesh, Brown Derby, Blue Zoo, Espn, Boatwrights) and they only charge you a dollar or two more, and they truly pour a double shot. I posted a thread about it recently...it is a great deal.
Have fun!