1.

def __or__(self, other):
    return self.__class__(self.maps[0] | other, *self.maps[1:])

def __ror__(self, other):
    return other | dict(self)


2.

def __or__(self, other):
    return self.__class__(other, *self.maps)

def __ror__(self, other):
    return self.__class__(*self.maps, other)


There are problems with both variants, so I think it may be better to not add this operator to ChainMap.