I don't know if their are any low pressure ports on the hydrostat, but I don't think that will cool very well. If you want to put a cooler on it I'd personally try and find one that looks like a little radiator/evaporator/heater core. Or if you happen to have one of them laying around you can clean it with something like Kooler Kleen and use it.
This is the type I'd get (this one is on my Town Car). I bought the cooler from NAPA for ~$50. It's designed for a truck that tows less than 10,000 lbs. This is in addition to the cooler in the radiator. In realistic terms I don't think you can get hydraulic fluid too cold. This works extremely well.
