In the past my wife and I have sailed several times during the Halloween cruise schedules and Christmas cruise schedules.
Some of those trips have either been Florida resident rates or Guaranteed rates.
We have also had some good luck finding those rates in the winter/spring time of the year when the kids are in school.
In the past most of the offers have become available,if at all, within the cruise line cancellation period where if you cancel your cruise you loose all your money. You also usually have to pay for the cruise in full when you book it.
On our last cruise we did a guaranteed ocean view. Disney ended up putting us in a deluxe family ocean view with a veranda.
In the past we have gotten the coveted double window large cabin for a guaranteed ocean view price.
Sometimes you get lucky.
On most of our sailing the reason they do this is, it is off season and they can't fill the ship.
On our last cruise we were told the ship was not full and it did not seam to be over crowded at all.
If you are going to take the chance for a merry time cruise, if you are flexible with your schedule, start looking in the time frame I mentioned above. Look at the cruises a couple weeks before Thanksgiving (just make sure it is a Merry time cruise) and the week before the cruise that puts you on the ship Christmas day.
This has been our experience in the past. Thing may have changed since then. You need to check with
Disney cruise lines if you are not sure how the offers, if any at all work and what is avaliable.
Good luck