I have the DDE card too and ended up purchasing the dining plan for the first time. I assume you know about the new "holiday pricing" at any fixed price restaurants? When I heard dinner at Ohana was going to be like $32 or something (I can't remember exactly) I thought it made the dining plan a no brainer. I think it will be fun to try and if we do need to buy anything extra OOP for the meals we share then I'll try to use the DDE.
Good luck and I hope you get a definite answer soon! I know if you ask Disney Dining they can tell you if there is anything in the instructions about whether the restaurant would accept the DDE.