If you book through disney directly, you can always add a dining plan if you book a package (room and at least 1 day of tickets). That includes ANY special offer. Dining plans cannot be added to a reservation booked through a wholesaler, so you would be correct.
DME can be included for any DIsney resort no matter how you book the reservation.
Also, if a
free dining promotion comes out for your dates (which apparently is the topic of discussion right now on here!) you would not be able to take advantage of it.
...just some things to think about.