Don't give up because someone else says you should.
But if and when you find a solution, make sure you tell us! That would be a great function for others and their application.
I wonder if you could build an external relay controller that only switches the fan when the rpms drop? But I'm way out of my depth here.
What about a thermostat on the spindle itself, switching the fan on only when needed - irrespective of rpm?
Cheers, Joe