I'd be inclined to wonder what the residual pressure is in the return line - should be close to nothing, but I'd consider it important... I know it's pretty much squat with the transmission, but peak operating pressure is much lower (300psi versus 1200-1500psi...)
Mainly, I'd want to make sure pressure is low so you don't have to worry about power steering fluid popping into the radiator uninvited...
Why not use an external cooler? Just quit using the oil cooler in the radiator, and remove the thermal load from engine cooling system altogether - that's good for about 5 degrees of engine operating temperature or so... Use a little two-pass cooler for power steering, a four-pass for the transmission, and put them both in front of the condenser. The lessening in cooling capacity due to reduced airflow is more than made up for by removing the cooling load from the engine coolant circuit.
As far as the transmission fluid goes, use some sort of thermal bypass valve - B&M made them, and I think FlowKooler does as well (and Hayden probably does too - check.) The power steering fluid just don't care - hydraulic fluids aren't picky about temperature as long as it's low - but the transmission likes to warm up quickly so the oil will work at maximum efficiency. The thermal bypass valve is designed so that the fluid doesn't go to the cooler while under 190*, and will go to the cooler above that - the idea being to maintain a temperature of 190-210* (well within the efficient temperature of automotive fluid lubricants.)
If you're going to do something like this, make it easy on yourself. Power steering depends as much upon volume as pressure, so you don't want to reduce the size of the lines (and while the internal oil cooler is plenty large, the entry where the flare fitting goes has about a 1/4" ID, IIRC.) That sort of restriction can cause backpressure you don't want to deal with...
5-90