In some application notes for MOVs I see example usage in power supply design where each input line (N, L) from the mains are protected with a MOV to a ground voltage reference (as well as a common-mode MOV protection between the input lines)

What I don't understand is what is the 0V "ground" reference these application notes are referring to?

It is both regulatory and practically impossible to use the PE (=FG) since even the smallest MOV current could trigger the earth protection circuit breaker.