Hi Piglet!
We have been to WDW 3 times with our autistic son (now 8), and our typical daughter (10). We use the GAC, but quite frankly, we don't need to use it that often. Yes, long lines can be difficult for all kids, especially those on the autism spectrum, but there are lots of things you can do to avoid long lines.
For instance, we always plan our trips during the less crowded seasons. Summer and Christmastime are simply out of the question.

There are also "tricks" to learning the least crowded parks for each day of your visit. May I suggest
www.TourGuideMike.com.
Back to the GAC. Ours states that he may use an "alternate entrance, if available". Of course, its the "if available" that is the operative phrase. Usually this pertains to the FastPass queue on attractions that offer FP. For all other attractions, there really is no way to accommodate this request. That is why it is best to depend on sensible touring practices rather than on stamps and GACs.
Getting enough rest, arriving at the parks at opening and taking midday breaks to avoid fatique and irritability are key. Knowing what factors trigger meltdowns and making every attempt to avoid them is also very helpful. Providing your child with familiar foods, toys, and "loveys" can help to provide security. Keeping to as many routines as possible will also be very comforting; if you brush teeth, then read a bedtime story and then say prayers before bed at home, continue this while at WDW!
Does your child utilize social stories? Make some up ahead of time and go over them with your child before your trip so he has an idea of what to expect. My son likes to do things in a sequential way. He likes to have a list of attractions that he would like to see in hand so he knows what's coming next. i.e. 1)Dumbo, 2)Pooh, 3) Peter Pan, etc. We also address the possibility of deviating from the "schedule" :
"The rides at the Magic Kingdom are fun."
"I like to ride the rides in order."
"Sometimes, however, a ride needs to be cleaned or fixed."
"It's O.K. to feel sad when I cannot ride a ride.
"I will ride that ride another time."
Another thing I find particularly helpful is to use the GAC for handicapped seating at the big shows, like Fantasmic! It is a blessing to be able to procure seats just before showtime and avoid sitting for a half hour or so while he gets more and more antsy. He uses soft ear plugs to muffle the noise of the fireworks and loud music. (They really help!)
There are other ways that the Cast Members can help you. When we arrive at the podium of a TS restaurant, we let the hostess know that we are willing to wait for a quiet table, preferably against a wall or window. When queuing up to meet a character, we let the "handler" know about our son's autism. The characters are always very gentle and patient with him.
I agree with SueM. Try to think about what your child's needs are and adapt yourselves to them. Every ASD child is different and I want to reassure you that you can advocate for what he needs very successfully at WDW. I have found that most CMs are extremely helpful and kind.
Feel free to check out my Pre-trip report on the trip report board, "KathyRN Family: Adventures in Autism-WDW." We leave for WDW on Nov. 6th. I'm going to try to include as much as I can about the special needs of my autistic son and how we work our WDW vacation around them!
Kathy
