Well here is an Option, perhaps not the exact solution you want, but will perhaps work for your needs......
Use OEMLED 166 (that is the "Abnormal Condition" LED), as long as there is not an abnormal issue, the abnormal conditon doesnt change state even under estop,
unless you have a internal abnormal condition. so you coudl use it as a "Mach3 is up, and in normal condition".......
scott