Hey Richard,
usually you control the VFD over the ModBus serial port on pins 13 and 14, so pin 23 ‘estop’ should still be free. Or do you want to say that you have used ‘estop’ pin 23 for something in conjunction with the VFD?
Yes, that indeed is not recommended, but Onefinity contradicts itself with the fact that the big red Emergency Stop button on top of the case is wired to simply cut the power to the controller at the internal power supply, as Onefinity stated here. In contrast to what you would assume, especially when knowing the Buildbotics Controller of which the Onefinity Controller is a fork of, on the Onefinity Controller the big red button it is not connected to “estop” (pin 23) functionality of the Mainboard. But of course you can wire it that way. Then after hitting the red button you would retain a running Raspberry Pi with the CNC Controller Application in “estopped” mode, i.e. with spindle stopped.
Further reading
Hitting Emergency Button on top of the Onefinity Controller box
Best way to e-stop spindle and VFD?
Estop Wiring Via Breakout Board