I have not heard of anyone yet hacking DJi's code, maybe I am wrong and it can be done but nothing on the forums i have seen. You run the risk of bricking it.
No firmware hack required. Simply cover the GPS antenna with tin foil and fly in Atti mode.
Also I believe with the upcoming GEO System firmware users will be able to "self-certify" that they have authorization to fly near an airport (>1.5mi), and deactivate geofencing themselves via a code from DJI's website. All you need is a DJI account which can easily be set up anonymously.
Similarly on DJI's larger, "pro" oriented drones / flight controllers (e.g., A2) one can simply turn off geofencing.