Sometimes it takes a little while to formulate an answer...
As I recall, liquified freon (R12/R134A) has a significantly lower viscosity than transmission fluid, so flow may be hampered through the "cooler" in this case.
Another useful little factoid is that the transmission cooler is usually plumbed into the fluid return circuit - AFTER it's done all its work. Therefore, there is little to no pressure - at most, it's just enough to keep the fluid moving. Therefore, system pressure is not a great issue here.
However, I'd not want to put the cooler for transmission fluid (which can be hotter than the engine coolant) in front of the heat exchanger for engine coolant - doing this can seriously reduce the efficiency of the engine heat exchanger (which depends upon fresh air intake to work.) You are therefore likely to see significant temperature increases in engine operation, which can lead to spark knock and other undesireable effects. Short form - Bad juju.
Would I do it? No, for reasons stated. I also don't know how the AC Condenser is constructed relative to the internal arrangement of the transmission/Power Steering cooler, and if it's different enough, you may not get enough efficiency out of the condenser AS A TRANSMISSION COOLER to make the effort pay off.
Besides, the typical four-pass cooler used with most automatic transmissions is sufficient - if you've deleted the air conditioning, remove the condenser and put the auxiliary coolers on the front of the radiator. You'll pick up cooling efficiency just from getting the condenser out of the way, and that will probably offset the occluded area of the auxiliary coolers.
If you are really worried about blocking the radiator, some creative thinking should reveal ways to use things like "frame rail coolers" (liquid/air heat exchangers with a narrow profile) to cool fluids in a manner that allows them to be kept out of the way - I haven't taken any time on this, but I don't see a reason why it couldn't be done.
For Very High Performance applications, the idea of using the condenser (after serious flushing and cleaning, of course) as a fuel cooler may have merit - but the viscosity of fuel is much lower than petroleum lubricants (and is roughly similar to water - or condensed freon) and should flow quite freely through the cooler tube. Also, the net heat removed from the fuel is lower than that to be removed from the lubricant, since it doesn't pick up heat from friction (which is a large part of what makes oils get hot.)
Given time I could probably think of a number of uses for old A/C condenser cores - but oil coolers aren't any of them - except where they can be used in a location out from in front of the radiator (so the engine can stay cool as well.) Anything else you'd like to try?
5-90
