It is possible to get the REF credit through Costco. I added it to my 2/22/03 Magic cruise back in May, although it took a couple of phone calls. I would definitely be persistent, as the quality of the Costco TAs varies quite a bit. And you have to add it through your TA - DCL wouldn't help me at all, except to tell me that the credit code was valid.
And as far as I know, Costco is still booking Disney cruises - at least, I booked one in May 2002. I believe Costco switched travel agencies last November, and that might have caused some confusion.
April