@jay_s_uk It may be if you know what your doing, but the configuration on this is done by Ooznest and I don't know what else would have to be reconfigured as a result of differences between RRF2 & RRF3, some commands are different and others have the functionality moved to a different M code.

@jay_s_uk I know, but that's no use to me as my Duet from Ooznest doesn't have RRF3 on it.

@dc42 Thank you, I had seen that I could create files for the unsupported M codes in the /sys folder. It does however seem very lacking for support of M3/4/5 codes when they've been implemented in to RepRap but don't actually do anything like switching a pin/port on/off to actually control a spindle. The only way to do this now is to modify a post processor to use the non standard commands for a spindle such as M7/8/9 which are normally used for coolant/air mist or M10/11 which are normally used for CNC axis clamping.
Thank you.

@droftarts I've had a look through some of this and the links you have provided, I can see how to output the PWM signal and configure it using M453 but cant see how to configure it to make M3/4/5 as well to control the ON/OFF of an output pin of my choosing. It would also be useful to configure other M codes to switch outputs for say a vacuum or air mist compressor.