Level Atitude, from Part-FCL (which has been read carefully):
FCL.070 Revocation, suspension and limitation of licences, ratings and certificates
(a) Licences, ratings and certificates issued in accordance with this Part shall be limited, suspended or revoked by the competent authority when the pilot doesn’t comply with the requirements of this Part, PartMedical or PartOPS, in accordance with the conditions and procedures laid down in Part Authority Requirements
PartMedical extract:
MED.A.020 Medical certification
(c) Applicants for and holders of a private pilot licence (PPL) shall hold a valid class 2 medical certificate.
So the CAA CAP804 definition below is in accordance with Part-FCL:
A “valid” licence. A licence is valid if it has been issued and: (a) it has not expired (or is non-expiring); and (b) it has not been provisionally suspended, suspended or revoked; and (c) the holder has a current and valid medical certificate or medical declaration appropriate to the licence
The bottom line is under Part-FCL and Part-Med you
must have a valid EASA medical for the EASA (and JAR) licence to be valid, otherwise you invalidate your FAA piggyback licence. The FAA have no power to decide if your EASA licence is valid - they just require that it is valid.