I suspect that it doesn't make a big enough change to bother doing.
You mention your humidity is 80% I assume that the temperature is high as well.
If your greenhouse was situated directly next to a good size creek you should be having cooler temps, no matter the humidity.
If you try to plumb cool river water thru pipes of any distance, then your water isn't going to be very cool anymore when it gets there.
So I think the fans on the radiator would be the same as just having fans in the greenhouse.
Like you say, I haven't heard any mention of this in a humid environment... I'm guessing its been tried and discounted as not effective enough.
Shade cloth does help quite a bit.
However that is just my thoughts on the matter.
If you give it a try, then let us know how well it works.