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.
To exercise any privilege under the ICAO recommendation, somewhere, you must have a valid ICAO licence containing that privilege. The recommendation only allows one State to recognise a privilege granted by another. That licence must also be acceptable to the State in which you wish to use it.
FAA "rendered valid" to fly the FAA SEP privilege?
You did not make it clear if you were talking about a FAA Cetificate rendered valid by virtue of Art 26 or a FAA "Restricted" Certificate rendered valid on the basis of a foreign (JAA) licence!