Eureka!
I found an even better way! The PS has a load share Pin with a voltage rising linearly with load. I used this to generade a PWM with the Arduino, which adjusts the Fan speed to load in real time. The code was finished today at 2am . It works excellent. So, with an investment of $2-3, everyone can have the best fan regulation you'd ever want.
More info and the code follow in the coming week