We do an early breakfast in the parks almost every day as my nephew is 2 and wakes up very early and very hungry.
We always check with the front desk the night before each breakfast (that requires a bus or boat) to see what time the buses (boats) run for the particular park (and then show up few minutes early). It seems that the buses do no wait around and we tend to eat around 8-8:15, so we make sure that we are present and readyto board. This has been successful for us at Wilderness, Yacht Club, Grand Floridian, and Animal Kingdom Lodge.
Lisacope -
You asked about where you have to stand if you finish your breakfast early.
at Animal Kingdom.
You stand outside near the restaurant in front of another rope. At about 9:03 AM, when you look behind you, you will see a huge group of people coming towards you. Everyone is heading to the Safari ride which is in front of you, but several yards away. It is somewhat funny/unnerving. The rope in front of you will be dropped eventually so that no trampling of you occurs. A recommendation would be to go to the Safari ride as well. You will be one of the first on it and the animals will be more active as it is morning. It is a wonderful perk with the Tusker House breakfast. By the way, we almost always forget that this is one ride where you get in line WITH the stroller and then drop it off later in line.
at Hollywood Studios
You stand near the restaurant or you can join the people in the "street" as they do a park opening ceremony thing. We joined the MASS of people heading to Toy Story after breakfast- this was the most chaotic to us, but not bad. A lady in another group did fall, so it slowed the crowd a little. The restaurant is not that close to this ride, but we still arrived at Toy Story with less than a 5 minute wait and we had slower walkers in our group
at Magic Kingdom-
After Crystal Palace you stand somewhat near the restaurant, but you have more space than the others- great pictures of castle, etc..
I don't remember EPCOT keeping us near the restaurant, but we stayed in the World section for a little while after breakfast..
There will be workers (castmembers) outside of the restuarant areas who will let you/not let you go...