Joe,
I have long harboured the idea that John of Hazel Green comes from SW Scotland. The only place in Scotland that has a name anything like 'Hazel Green' is a small place a few miles SW of Newton Stewart not far from Garlies Castle the home of the Garlies Stewarts. There are plenty of Johns in the family.

I would guess the ballad is not very old, perhaps early 18thc. I base this on the peaceful nature of the ballad, a not very common occurrence in pre-Union Scotland, and the fact that the father can roam at will and pick up girls on the roadside.

Are you anywhere near Newton Stewart?