actually i was going to mention something to stillen about this but forgot about it until now. Most efficient way of using co2 in a water to air setup = tank bubbler. Think stainless co2 bar submerged in the inercooler tank just have to make sure there is enough antifreeze in the mix to keep it from freezing
Edit forgot to mention that this way it is a direct cooling effect from the gas to the liquid instead of having to go through the metal of the cooler, and it contains the cooling effect of the co2 to the water more. I'd insulate the tank, and make it a good bit bigger then the one in the previous photo i saw, to maximize the time after each burst of co2 before the water would heat soak again. You would have to come up with a vented reservoir with a kind of catch can to keep the coolant in but let the co2 out as you would want the gas to flow directly through the water instead of just using an expansion chamber in the tank(think a/c evaporator) for the greatest cooling effect