I would think that the main reason to foam a roof would be to allow extra working time for the product. This may be fine on a walkable roof,...but when you really need it to stick extra good is on the steep roofs when runoff is the main concern. But considering these roofs are also not walkable it seems to me that shooting the product from the roof edge with foam would maybe create an issue with getting any distance with foam,...plus I think it would be way to susceptible to even the slightest breeze.
Jeff