Depending on how quick you want your visit to DD to be, I would think it would be pretty difficult to try to fit it in.
We rented a car after our recent 4-day cruise (3/14). I may have the time totally wrong, but I think we were allowed to debark starting around 8:30. There are two seated breakfast shifts, we were in the second one, but opted instead to have a very quick breakfast at Beach Blanket Buffet so that we could get going and get off the ship more quickly (we had also decided to carry our luggage off on our own and didn't want to take it to breakfast). As debarkation time neared, a long line--that would move and then stop for a while, move and then stop for a while--had formed. When we got to the line (pretty early), it was already back to the Internet area of the deck. After about 1/2 hour of waiting we got off, strolled quickly through customs (since we had our luggage, we didn't have to find/retrieve our stuff) and then walked over to the area of the parking lot where the rental car shuttle buses were picking up. We waited for the
Avis van for about 15 minutes, then went right to Avis (a few minutes drive) picked up the car with no waiting, made a bathroom stop, and headed to WDW for the day (we had an 8 p.m. flight).
We were among the first people off the ship (it seemed like many did opt for seated breakfast), got the shuttle to Avis in a reasonable amount of time, got the car quickly, and had NO traffic. I think we arrived at the MGM front gate at about 10:20, maybe a little later.
Assuming that you'd want to be back at MCO sometime shortly after 11 a.m. (and considering that it probably takes at least 30-40 minutes to drive from WDW to MCO) you'd probably have almost NO time at DD. Of course, if you already had your car parked at the port (and didn't need to catch a shuttle and pick it up) that would save about 1/2 hour and, if determined, you could eat first thing at BBB and be the very first on line to debark . . . so, total, that might gain you almost an hour to spend at DD. I guess it depends on your situation and desire to drive back and forth and rush a lot.
Returning our car to Avis a MCO was actually pretty painless and quick, but it still took several times around the airport to find the rental car return area (really poorly marked!) and, of course, it wasn't as quick as just getting dropped off at the terminal (had to return car, walk to terminal, etc.). Not to mention driving around the area to find a place to refill the gas tank (or face the $4+ per gallon fill up charge!).
I think it would be very hard and VERY stressful, especially considering the increased advance time needed at the airport these days), but it might be doable if you were very serious about fitting in 1/2 hour at DD.
