I'm not opposed to the idea, but just wondering if its needed.
If the car is idling at the lights on a hot day, the trans isnt really generating a lot of heat (this occurs when you torque it up)
If you already have a trans cooler it would have better cooling than a standard car, (and stock autos dont have many problems) and I would be a little worried that the fan might restrict air flow and in effect make it worse when you are driving, and when the trans IS making heat.
I am a big fan of trans coolers (I have one on my auto) but im not sure if the fan will offer a lot of benifits