I use a big SSR (solid state relay) on my VFD. It's controlled by 24v switched by my Mesa I/O card. The VFD doesn't have power until LinuxCNC is up and switches the bit on the I/O card that enables the SSR to power up the spindle VFD. In case of E-stop, it is (actually "will be") set up to cut spindle power and apply the brake.
Erik