I think this will be a tough find. Keep in mind that DC caps the number of people per day, plus is all inclusive. I don't think they need to give discounts, and extra foot traffic through the gift shops is not enough to justify a dfiscount even on slow days (if there are any). I have been there during the first week of August and in early November, and it never felt crowded, even though there HAD to be more people on the summer trip. That's the beauty of DC...relaxation!
I did book a package through Sea World that included hotel and SW/Aquatica tix and DC offered me $40 off per person, since I wouldn't need the same unlimited SW/AQ passes, but I needed my vacation reservation # to get the "discount."
But hotel and only DC tix as a package with any inherent discount just doesn't seem to pan out for their revenue model.