From what I can find, ATF has a specific heat half that of water. Of course not all ATF is created equal, but this estimation is better than nothing.
Looks like 0.5 Cal/g°C is 0.5 Btu/lb.°F from two different specific heat conversion calculators I tried.
Maybe one of the aftermarket shops who works on the zf transmissions would know the flow rate of the pump.
You would still need to know the temperature differential between the hot fluid and ambient air to calculate BTU/hr through an air oil cooler, right?