@John Wells That is a cool idea. The arduino does have an eeprom, so a value could be stored there in case of power failure or something. Someone would have to write the code, but we have a couple of coders on here. Maybe one of them would be interested.
I currently use movable endstops, which work for milling but are problematic. If I forget to move them after setting the zero points, they ruin the cut, and I do forget them at times.
This would make having some permanent endstops really useful.