I have 2 with HFAutism
My DD7 is "all about the RULES" and can basically occupy herself "playing movies in her head" infinatley
However...
My DS5, while not nearly as "autistically minded" (he has more understanding of other people and he makes more natural conversation), he has many more physical problems than his sister (apraxia, very overwhelmed by noise, crowds, lights, smells, etc. - he either melts down or stims his little head off) would not have been able to tolerate a day in a park without our trusty GAC -the Guest Assistance Card
You can get the card at guest relations in any of the 4 theme parks. I actually forgot our letters of diagnosis (left them in the copy machiene at home), and when I began to explain the situation to the CM in Town Hall at the MK, she took one look at my kids (stimming their heads off, of course!

) and wrote the pass!
Without that card, we wouldn't have been able to see/do 1/2 as much and we would have not lasted 1/2 the day.
We were able to do 2-3 attractions in a row then take a break at a playground/resturant and repeat.
I'm usually into "toughing it out" and I've made alot of progress pushing them to face their issues/fears just to their thresholds (without going too far and really upsetting them); but, I'd rather them just be able to have fun!
With the GAC, the trip actually IMPROVED his function (serious sensory integration therapy), whereas without GAC he would have been more stressed out than before!
I thank Disney sooooo much for this as we really wouldn't be able to go without it!
Just take whatever documentation you have and explain the situation, that your DD just doesn't have an "official" diagnosis, yet; I don't see why this wouldn't work.
Hope this helps,
IncredibleMom