The one time the matter has come up in my life, my best friends' son spent a week in the truck with me and a week at my home a few states away from his home. When we went shooting at the local public range, the Range Master, an off duty deputy, noticed that the kid addressed by my name, not Dad, and asked our relationship. The RM declared that I was "legal guardian pro tem" as the kid's parents had given him into my care for the trip. A valid legal opinion? Perhaps not, but it has never come back to bite anyone.
Gee, I'd love to see your data!