I did make a Brain for it, and can turn it on and off. However, it's got no feedback and won't shut off if the VFD has an error like overcurrent. It can't even tell if the VFD's breaker's off. So, it's not very safe.
I could add a test to the VFD's RPM register and stop if they disagree but I don't think it'll work, because once I issue a M03 command to start the spindle, the RPM will be 0 or very low initially and you don't issue Stop for that. I don't know if I can write a time delay with a Brain.
It would be more elegant and robust to have a plug-in, the Huangyang one looks good. And Huangyang is a very similar communication protocol.
I also would really like the Spindle RPM field to work in Mach3. I don't think it can, it appears to be hardwired to the tach input and trying to write it with a Brain had no effect at all.