I think the problem is terminology. If the meter fitting has a main, then the next piece of equipment is not a main panel but it is a downstream panelboard.It is pvc, the GEC would land in the meter base which has a main breaker. The reason we want to do this is to provide a GEC for the main (sub) panel.
OOOOOPPPPSSAnd in your last post I think you meant to say . . . .