I am building a BLDC driver for a non-cnc application, but would like to convert it to step and dir use as well.  Is there a method in Mach3 to wait for a "Move Complete" pulse from the driver?  Having never used a servo before, it just seems likely that each position (from the step) would need to be reached prior to sending a new pulse.  Is this correct thnking?

No,in the parallel port version Mach sends out step pulses and just assumes the controller does its job and makes the move. There is no feedback to Mach.

With devices like the Grex (G100), ncPod, and the gailil (sp?) where they are driven by a plug-in this can be different, depending on the device.

If a USB device was emulating a keyboard, the processor could return a pulse to pause Mach3 on each  step(as USB key command) followed by a continue command(USB) once the move was complete.  The question would be how slowed down things would get.  But, is this at least doable?

How hard is it to add the pause/continue update to the parallel port and software?  My old MAXnc10cl would do something similar I recall when using MAXNC CL mode.