Isn’t this just a Digital Fletcher-Munson loudness knob?
Absolutely not! The only thing Har-Bal does automatically is to apply a level change to you track to compensate for the change in perceived loudness caused by your filter design. It is no different to a mastering engineer with conventional gear using his master fader to lift or lower the levels to compensate for changes made by the EQ stage. The only difference here is that Har-Bal does it automatically for you.