I have just a small idea, perhaps not objective: I know this plant for just 2 years. Last winter (2012), the temperature felt -14°C in Marseille-center of Town. These low temperatures are such exceptional here! (My orchard is located in zone 9b where usual extrema in winter is around -5°C). The Zagros plantules were located in a container, just beside a south oriented wall concrete-made. In winter, the solar irradiation is weak here, only one hour around 12h, so the thermal storage is nearly "0 Joules"!
Result: The young Zagros fig trees can cope with low temperatures. They have been no stressed, nor disturbed by coldness.
Therefore, I confirm the containers was set beside the wall, and directly on the slab of the terrace. It is important because there is no vaporisation of water, thus dry air. The Maine is not Arizona! According to me, you absolutely need to set zagros on rocky places, or keep them in container on your terrace.
Cheers
Chriss