After reading some more, I have an idea.
The stock "cooler" setup is an ATF/water heat exchanger. During warm up, the coolant is warmer than the ATF, and heat flows from the engine coolant to the ATF to bring it up to operating temperature faster. Once the engine is hot, the ATF is warmer than the coolant, and heat flows in the opposite direction.
So my idea is to place to an axillary cooler before the stock heat exchanger in the ATF cooling circuit. This way, cold ATF would flow through the aux cooler, then to the exchanger where it would pick up heat from the water on its way back to the transmission. Once everything is warmed up, hot ATF would flow through the aux cooler first, lower its temperature, then it would flow through the heat exchange where one of two things would happen. If the ATF was still hotter than the coolant, heat would flow into the coolant. However, as long as the cooler is sized properly, the ATF should be below the water temperature, in which case the ATF would be brought to the coolant temperature and return to the transmission.
Regardless, the ATF would transfer less heat to the coolant with an axillary cooler placed before the heat exchanger in the cooling circuit. The greater temperature differential between the air/ATF with the aux cooler compared to the coolant/ATF heat exchanger should result in improved cooling of the ATF, as well. This depends on the size of the aux cooler, but with the aux cooler before the stock heat exchanger, you shouldn't be able to make it too big, even in northern states.