I finally wired and configured Mach4 to use the start / stop buttons that I installed along the emergency stop button a while ago. Moving all the home / limit / probing switches to SmoothStepper Port 3 freed inputs on Port1 than I was then able to use for the buttons. The start button that I used is recessed, making accidental activation less likely.
I had to change the screen script to call CycleStart / CycleStop on inputs activation.
All in all, I’m pretty happy : The buttons allow me to look at the machine instead of mouse / screen to start / stop the machine