Some posters have reported doing just that. As long as your Emerald Aisle reservation is for a full size vehicle, National shouldn't have any problem with you taking a car from the full size aisle.
In fact, Emerald Choice service is also available at the Orlando airport. With Emerald Choice, you just go to the aisle for your reserved car class and choose from the cars located there. If you are really worried about going to the full size aisle when your reservation says Emerald Aisle service, just make the reservation with Emerald Choice service to begin with.
Enjoy your trip!