USMarine, I have a similar setup and will be doing the same thing soon. I did some research to ensure that I'll be able to rigid tap, and get the full value from the Servo spindle. The best method that I have found is not to use the typical SwapAxis method, but rather simply using BOTH the Spindle step/dir pins and the A-axis step/dir pins both set to the same output ports/pins in Mach3. The wiring will be just as you did XYZ with step/dir wires feeding from your BOB to the DMM driver's pins 11 & 22 (5V) and pins 10 and 23 (output pins from your BOB) . When you need to tap, you'll use A axis G-code.There is a video on Youtube demonstrating this method, and it appears to work well.
https://www.youtube.com/watch?v=vAwoXs2c6GY
There may be better methods, but this is the best I've stumbled across and what I plan to use once I have time to finish up the servo spindle motor mounts, etc. Chime back if you find a better method, many of us would appreciate the guidance.
-Mike