Thanks for your lateral thinking observations re the hay bale floorloading quandry. The barn I was designing was the replacement for the burnt down barn at Northfield Mount Hermon School, Mass.
I managed to get around the problem of overloading of the mow floor by inserting a loft floor as well. This physical barier limits the static head pressure which can be exerted by the hay on both the mow and loft floor joists. I have come to the conclusion that hay bales are better stacked on the ground up through an open barn structure as was the original practice and yes this would help with stablising an old barn.
Looking back to see the way ahead !