we decided to connect pin 9 to iButton contact, the 1-Wire bus
That way the wire #9 should be labeles DISTINCTLY different as a warning.
Personally i would remove the “9” number at all. Or at very least, rendered it into inverse white-over-black-mask colours. The glance on the panel should instantly conway “there is something wrong with this pin, something unusual, something that needs urgent RTFMing before going on”.