so who issues this rating and under what authority?
Chuck, It's an EASA thing:
FCL.815 Mountain rating
(a) Privileges. The privileges of the holder of a mountain rating are to conduct flights
with aeroplanes or TMG to and from surfaces designated as requiring such a
rating by the appropriate authorities designated by the Member States.
It's probably less restrictive than the old French altiport rating and is at least everlasting if you maintain currency.
I'm curious as to how you would go about flying to one of these places with a non-EASA ICAO licence that doesn't have an equivalent rating.