Update on this topic:
The Zero % Override setting from the System/Config menu is only working in combination with the Override Select function (MENU > SETUP > Ovrd Select). When Zero % Override is set to TRUE and the Override Select function is configured and enabled, it is possible to send an override of 0 and stop the robot without dropping the Hold signal. The system variable $MCR.$OVR_ZERO indicates if the override is 0 (TRUE) or <>0 (FALSE).
BTW: From software version 7DF3 (V9.30P/18) or later and 7DF5 (V9.40P/15) or later, it is possible to configure a GI for the override value.