After reading all the posts, the one thing that kept running through my mind is to use something that would not harm the child if swallowed. I keep picturing pockets that you could insert weights into perhaps with a velcro closure. I do not have autism, but I enjoy heavy blankets in the winter. Another possibility are large washers sewn into the quilt. I'm not sure about the cost, check at a hardware store. I keep picturing a well loved quilt letting the pellets fall out all over the bed.
Sue