Familiar with Art 26, thanks, bookworm (despite my typo in the original).
So we seem to be se settling on the interpretation that you CAN use the foreign licence for ICAO privileges for a rating you do not hold on your own JAA licence.
I know the FAA specifically prohibit this with their version of the "rendered valid".....if you have (say) a PPL SEP FAA full licence and a JAA with SEP and MEP, you cannot get a 61.75 "rendered valid" approval to fly MEP - though you CAN have the instrument rating privileges extended to a US licence.
Wouldn't it be nice if there were just one rule making body....call it something like International Civil Aviation Organisation.
Thanks for the replies.