I often dream up stuff when I sleep and this one I think would be ideal!
I noticed that when I physically square my machine, the bearing hits the corner posts of the MPCNC frame because it protrudes slightly from the mount its in. If you put a contact there for the bearing to hit (like a bolt, plate, paper clip, whatever) attached to one side of an endstop circuit (in Normally Open mode “NO” or true in the firmware), then the other end to the rail that the bearing is on (I assume current will flow through the rail and through the bearing races and bearings inside), do this for both ends of each axis in series, then the circuit would only close when both ends bottom out. Voila! Endstops and squaring all in one!
Have yet to try it but I think this would work. I’ll let you know if it does if someone doesn’t beat me to it.