I asked around a little and so far, no-one has mentioned such a method when trying to list possibilities.
Considering he is a veteran who served in Afganistan, then could this particular method be something he witnessed there? Say, is it possible that someone (his army mates, Taliban fighters, local troops, anyone) used this method on local kids? As most war crimes go unreported, it is unlikely we'll get a full picture, but I'm thinking more generally, that is this method "a thing" at that time and place where he served? I found some hints that it might be, but nothing of the degree I was expecting for "confirmation" that it might be. (I'm saying nothing about intent here, if this is a method he "picked up" in Afganistan, he could have used it in either "sane" mind or not).
So googling around I did find this article:
On November 24, an interpreter working with German troops was killed in Kunduz province, in northern Afghanistan. His body was found inside a vehicle with a plastic bag over his head and his hands tied behind his back with wire.
And this one:
Al-Ani was captured by Joint Special Operations Command troops in July 2003. He told investigators that during his capture, he was forced to lie on his stomach in the back of a vehicle with his hands bound and head covered in a plastic bag. According to a letter al-Ani wrote, his captor “put his foot on my back and started screaming and cursing me in English, which I do understand.
And this one:
After the Afghan children arrived at Samaritas, Grand Rapids police responded nearly every other day to calls for incidents like missing persons, suicide threats, fights and assaults. The police reports were unavailable, but internal shelter records document many of those incidents. One boy put a rope around his neck, “acting like he wanted to hang himself.” Another day, a boy tried to suffocate another child with a plastic bag. A few days later, a worker found a boy scratching his forearm. He told her that “when his body is in pain, it prevents his head from thinking about his problems.”
