Actually I dont think it is polyurethane,more likely a polyethylene,sorry,my sloppy posting there..
I am wondering whether it would actually be simpler to weld localised flanges to the stringers and 'risk' the localised corrosion of screwing self tappers directly to them,minimised by dipping each s/steel screw in sikaflex..?
Here is one link anyway to a supplier of Kingboard with product description

King Board