THIS IS HOW YOU DO IT!
My wife and I catch Fantasmic every couple of months, and we get a good viewing area without having to wait in line or pick a spot two hours in advance.
We make a reservation for Cafe Orleans for 7:40pm, requesting an outside table (waiting a little past our reservation time if necessary) and eat dinner. Most of the tables, even outside, don't have good views of Fantasmic. But people leave after dinner, and they stop letting people in. We then move to another table (which the wait staff helps with and even encourages). The one in the front corner has the best view.
Cafe Orleans will let you stay there during the show if you've had dinner, but they won't let non-diners on to the patio, so there's virtually no competition for seating. One caveat, though, is that when the show is beginning and all the lights go out, they're required to remove all glass dishes from the patio, so make sure you're finished eating (though they will transfer your drink into a to-go cup for you).
We get a nice view every time, dinner right before, comfortable seats, and we don't have to wait idly by for the show to start; or deal with issues of whether or not it's ethical to save seats for other members of your party while they're off doing other things.