Yes... this can easily be done. When the package is officially released, simply call CRO (or your TA) and ask them to apply the discount code to your reservation.
One misconception is that if you've already booked a certain room category/resort... you'll automatically be able to apply free dinning with no issues... and this isn't always the case.
With every promotion, Disney offers a certain number of rooms and room categories... meaning that when you call CRO... your room category/resort choice may not be available with the promotion.
Not to mention that due to the poularity of this promotion, it's not unlikely to see resorts sell out for certain dates. In 2007 rooms at Pop were "sold out" for certain dates around 11am on the day the promotion was released to Disney Visa Card holders (3 days earlier then the general public).
My one suggestion would be to have a back up "just in case" you can't apply the discount to your reservation/room category (it's not a common issue, but possible, so just be prepared so your not put on the spot!) and to act fast! The sooner you call, the better your chances, especially if your dates are more popular... such as the last week of August/Labor Day compared to the middle of September.
We prefer to book when the package comes out as our resort choice is very flexible... but if I wasn't going to budge on my room choice... then I'd book now... and try and apply any discounts that are valid for my vacation dates.