Well, here's the thing - propylene glycol, and some of the other alcohol variants used in antifreezes and cooling system additives have a low vapor pressure and a high boiling point, and have the affect of raising the water's boiling point as well. As well as to lower the water's freezing point.
The problem with lighter alcohols like isopropyl or ethyl alcohol, is that they have a high vapor pressure, and a low boiling point, which would make cooling system internal pressures even worse than with just straight water. Both are also very corrosive, and would probably lead to an early radiator failure, as well as plugged coolant passages over time.
Isopropyl alchohol might be used as a trace ingredient in some of the cooling system additives, but if you were to use it straight in any significant quantities, your are likely to have problems.