Somebody on this thread was recently given the option of red velvet for a Contemporary Bakery cake, which I've never seen offered there before, so you may be good to go at the Wavejust call Private Dining at the resort.
Otherwise, the Grand Floridian can do red velvet, but you may have to be assertive about it. Sometimes they are not as accommodating about doing non-standard flavors like the ones they offer for wedding cakes. Also, just to warn you, Disney's red velvet is not that great IMHO. But if all you're after is the Christmas color, you should be OK!