I'd have to go with the Hydraulic method of pushing a dent out of any tube - any fluid inc. water would achieve the same effect though clearly anything oil based is going to have the added benefit of corrosion prevention. Biggest risk surely is that anywhere but the the dented tube is going to push out, especially with freezing water. I'm sure would work equally well but I'd advise real caution if your going to try this one. Sticking it out overnight into the cold and waiting to see what happens without any form of control over the process is a sure fire way to end up with a neat split on the tube somewhere. I've seen 3000 hp Sulzer diesel engine cast steel cylinder liners almost split from end to end as a result of frezzing coolant so I'm sure it would make light work of a frame tube if left freeze uncontrolled.