It is recommended you make reservations at least 30 days in advance. That way you get your travel vouchers and luggage tags before your trip.
Having said that, you are only REQUIRED to make reservations before you land at MCO.
I asked this question of DME directly.
One of these days, not sure when, but I have a goal to wake up on a Friday morning, go to work, book Disney resort, airfare, and DME to arrive at WDW on Friday night, then return home Sunday night. I'll run to the house for lunch and pack a few things, then head to the airport after work. It's a bit more difficult now that Delta just totally changed the times of our two direct flights from our local airport, BUT SW has an 8:30pm direct flight on Friday nights, and an 8:30 pm flight back on Sunday nights, so that may work.
(It's my goal to do this before my AP expires on 10/28). I called DME to see if I was eligible, and they said yes.