Adding an LED at the probe could be made to work if you put the probe
on the input for pin 15 and set the jumper at JP5 to the on position.
This will provide a stronger pull-up resistor so that you can connect
a series connected LED and current limiting resistor across the probe
input and still have the input work. This arrangement is used in the
"WildHorse" probe and the input will work with it.
http://www.wildhorse-innovations.com/support/EPHookup-Alignment-new-v1.pdfIf this is not practical for you, you could modify your Mach3 screen set to
put and indicator on the main screen.