I don't know the answer to your insurance question, but we're still in the relatively early stages of potty training our globally-delayed SPD 4-year-old. A nurse at Children's Hosp recommended her go-to potty training book "Toilet Training for Individuals w/ Autism or Other Developmental Issues" by Maria Wheeler. I've read about 2/3 so far and have gained some great ideas including picture charts/schedules etc. There are many vignettes of children (some teens) that were finally successfully trained by using this or that approach. (I know this wasn't your question, but I thought I'd toss it out there anyway).
Good luck - I hope you get a positive response from Aetna. We also have Aetna and with a couple of letters of support from our pediatrician, we've (so far) had a positive response for OT, PT, and speech therapies.
Edit to add: I asked my husband about your question, and he did some additional research regarding coverage. He'll take over typing from here:
I can't speak to Aetna's policies, and because I was just laid off, won't be able to ask them until our COBRA coverage kicks in in six weeks. But here are three links which address your question form the point of view of different disorders/diseases, including neurologic ones such as autism. The take home form all seems to be that(1) if your child is enrolled through Medicaid, it is almost certain that diapers would be covered with a prescription from the pediatrician. (2) private insurance covers diapers less frequently, even with a prescription. However, the bellaonline article, written by a mother of a 9 year old with diagnosed autism seems to have had great success with her plan. I am actually uncertain if the plans she describes are Medicaid or private insurance, because she is writing from California and I am in Washington. But they are being covered, either up front or by reimbursement.
If your claim is denied, the second article suggests finding a pharmacy that will bill under general medical supply codes, which seem to facilitate coverage.
http://www.bellaonline.com/articles/art55755.asp
http://www.articles.complexchild.com/jan2012/00356.html
All our best to you on your search for answers. We are interested in knowing your outcomes.
D. and Dave P