If all else fails, you could go to an RV dealer and inquire about the "enviromentally safe" Anti freeze used to winterize RV's.
Mix an appropriate amount with the water and pump it out in spring.
Dangers would be any deletarious chemical effects to the FG.
Probably not a problem since modern car radiators have plastic fiberglass tanks on each end, but there are different types of plastic. You could mix a bit of fiberglass with antifreeze and see what happens.