They do and we've done it at that time. It was actually a very nice change for us. We are doing it again for our August trip.
So on the day we had our late lunch, we had a late big breakfast. I think we did the cape may cafe so we were really not hungry at a normal lunch time (12-2) then we had our early dinner at 3:30. Now we don't do the ddp so we ordered appetizers and a meal. What made it really nice was about the time of fantasmic our stomachs were just empty enough for us to enjoy a really nice dessert.
It was also really nice not to feel bloated after dinner. some times when we eat at 6:30- 7:00 ish we still feel full walking around the parks at night.
To answer your question, I think you still have to check in about 45 mins before hand.
The fantasmic packages usually have early seating times for meals. Thats why they do the package, to fill the restaurants during quiet times.
you can keep checking back and try for a later time-you can do this online.
what you do is go to your meal and enjoy. your server will give you a ticket for the Fantasmic show. when youve finished your meal you can explore the park and then head to the fanstasmic show later.
With your ticket you can by pass the regular queue