Wiring of servo spindle - MESA 7i76E

deus wrote:

• What is the typical fault signal called within Linux CNC that would be good to trigger the physical e-stop loop?

Possibly halui.estop.is-activated but that is from halui so not real-time. 
linuxcnc.org/docs/stable/html/man/man1/halui.1.html

I would start by looking to see if motion.motion-enabled has the desired behaviour:
linuxcnc.org/docs/stable/html/man/man9/motion.9.html
That's from the motion module, and should be 100% reliable even if the GUI crashes. 

You can use halmeter to watch how pins change state. 
linuxcnc.org/docs/stable/html/hal/tools.html#_halmeter

You might also need:
linuxcnc.org/docs/stable/html/man/man9/estop_latch.9.html