My son has encopresis as well. Also diagnosed with Asperger's and ADD. I understand how difficult it is. My son Is often in pain. I had a pediatrition who took it very casually for years. Once, by chance, we had to see a different pediatrition who I spoke to about it. He was ready with a plan. I was sooooo relieved that he took this problem seriously. My son is eight and had been on mineral oil since 3yrs, then benefiber, senekot etc. The first pediatrition only delt with it when it was an immediate problem. Now the second guy is making sure it gets the attention it should. Even though his advice of Miralax produced no results, he sent us to a GI specialist. My son is now more comfortable and doing better, but it is an ongoing process of treatment since the muscles need to gain back their elasticity from being stretched out from the withholding.
Yes, this is more common for Pervasive developmental disorders or ADHD.

It is hard to get people who know about it to admit it.
When taking out books, or researching information, I found people who shyly admitted to me that they knew of this.

Hope this helps!! It helped me, thanks!