You could sell a red mushroom/panic/emergency-power-off button the user can smash to stop it…? I put light switches on my maslow and everything plugs into outlets connected to the light switches so it can all be shut off quickly rather than unplugged.
The M2 controller has a bad history of doing this when there is no machine home set. Some go up and some go down, but they will go even if just moving the z axis unless homed/reset chains first.
You are talking about a beam break sensor like the garage door eye? what if you accidentally put your hand up there while in the middle of a cut? You would have to add logic to the firmware to incorporate the sensor, so perhaps the root cause could be addressed in firmware? The assumption could be that the chains are extended at the center point rather than at full extension since manually mounting the sled will put it near the middle anyway. Before calibration, we need basic movement, so having the system set to think the sled is in the middle would be a reasonable starting assumption. Thoughts?