Hello Frareg,
I don't have the ESS Smoothstepper, so I can't test it myself, but from its description I understood that this is a CNC machine control board with Ethernet or USB interface, which connects to Mach3 using the plugin. As I read its description, the Smoothstepper has an output for Spindle, so you can use it with my device. You should set the spindle to PWM mode. Use the same frequency of 350 Hz, as for LPT device. This should work. As the brushless controller has its own optocoupling, you can connect the converter directly to Smoothstepper.
Please let me know if you have any questions and the results, of course. It would be interesting!
Best regards!