This
is what you get on a Sandel
SN3500 EHSI if you have the course pointer pointing backwards instead of forwards
A Back Course indication, and glideslope suppressed.
A normal KI525-type HSI, or a basic CDI, couldn't care less what the CP or OBS are set to. However, if you are driving an autopilot from it, then it may well matter because in NAV/APR modes the AP uses the CP as a "heading bug" and uses the deviation of the deviation bar to modify the heading. So that's another reason to set the bearing to the right value, though I am sure this will be system dependent.
Is there any way to emit a localiser without emitting a back course equally - using the standard type of LOC antenna array which runs across the width of the runway?