Given the knock code doesn't actually indulge in learning, could the input for it be subverted into doing retard for nitrous? I doubt it would be a serious code change.
I have wondered about nitrous on the Griff - I'm not going to be allowed the supercharger I want after all - and thought about a dry system (I have daft injectors in it) controlled by the PWM from the boost solenoid output and mapped such that the system progressively applies the nitrous through the rev & TPS range. Should make an exciting (but illegal) road car!