You can actually call DME and book the day before, but you won't get your luggage tags. You need to give them at least 10 days to print out the info packet with vouchers and luggage tags and then get them to you.
I would book it, with info given for a possible flight that you might take. DME doesn't really care about exact flight info...they just need a ball park figure of how many people are coming into MCO at what times, so they can provide enough buses.