I wondered this myself and enquired at Ashbys. They reckoned the alcohol content would'nt do the pump seals any good.
It would have to be a weak solution but that would defeat the object. My van is kept in the street and most times I can't park near the house, so what I do now is...on the last job I empty the tank through the hoses incl wand, hand tools till all water is 100% out. So far in the coldest frost I have had no problems.
