It's official: Brad is day-trained. He's been in undies for a week, with only one accident. Here's how we trained him:
Every weekend since the Countdown began (about 6 weeks ago), we gave him two hours of underwear time. On Saturday, I would make sure he had a success, basically by keeping him on the potty at the right time. I figured out that he pees at around 8:45 AM. When he "succeeded", I gave him an M&M. On Sunday, I would give him underwear time, but just let him do his thing, which caused him to have an accident, in which case, no M&M. Also, during the week, we started doing potty time after breakfast.
For the first four weeks, his weekday potty time (after breakfast) consisted of him sitting on the potty for a few minutes and not peeing. By week 5, he must have figured out how to hold it in because he started peeing in the potty after breakfast.
So that's pretty much it. But every kid is different, and certainly spectrum kids have body awareness issues that typical kids don't - what worked for Brad might not be the answer for everyone.
There's also a normative parenting issue embedded here - some believe any form of potty pressure or self-imposed timeline is wrong. I see it the reverse. I feel as though independence is a great gift to Brad - indeed, he smiles ear to ear when he potties - and as a parent, I feel obligated to at least give him that chance.