.
Its only illegal if you couple it with airspeed / GPS input from the plane.
.
I don't think using the governor inside the Castle ESC would be useful either , you only get three (four if you count zero) selectable RPM's, which would be quite a handful to manage during a pattern sequence (but entertaining, yes!!).
.
However, I believe that the usefulness comes in when you can use the governor to select RPMs from 0 to maximum with the throttle stick instead of power (stay with me here, there is a difference!!). You would have to do some data analysis of your flights and determine what RPM your batteries can give you at the end of the flight, then program the governor to limit the RPM to that at the beginning of the flight so you can achieve a consistent response thru the entire flight.
.
I could even see a system where your governor would "learn" your power system over several flights and then set up its own RPM range based on a past history of voltage drops during the flight. Of course you would have to use batteries of similar age, manufacturer, C-rating, capacity for the learning to be valid. But eventually the governor could learn to limit the initial RPM to where the flights would end with enough "headroom" to keep the performance consistant thru the entire flight. Then from there you could program either a linear RPM or RPM curve that is commanded by the throttle stick.
.