I have read comments from people that are staying in a Deluxe resort at the Concierge level, where the Concierge planning CMs were able to pre-arrange participation in the Flag Retreat Ceremony.
So it seems to me that they do pre-book it. Call Guest Services and see how far in advance they can do this. I would not wait, as it seems like a lot of people would like to do this for a spouse or parent that is or was in the service.