I agree - Foam quality or even foam type (eps vs. poly) doesn't have much to do with how easily a board will snap. Zach - if you want to keep riding light, flexible, custom shaped poly or eps foam boards like most of us want to, i really dont thing there is much you can do to make them resist snapping, that wont sacrifice performance.
glassing schedule mostly and things like stringer material, and rail shape relate more to a boards flexural strength (snapping resistance). heavier glass or boxier rails will def make the board more resistant to snapping but you wint like the feel.
There are supposedly types of 4 oz. glass (warp bias) that will not increase the overal weight of a board, but will make it harder to break because it contains more fibers running lengthwise. Heres how onw manufacturer explains it:
"Warp Bias Fiberglass - contains more yarn in the warp (length) direction for the purpose of added stiffness and buckling resistance"