I'm only an EASA FI, but if someone would ask me to sign off a FAA Flight Review based on the bi-yearly SEP revalidation, I would politely decline that as I have no FAA qualifications. Looking at the intent of the regulations, I read this as:
1. You need to keep your non-US license valid, including the bi-yearly revalidation requirements. As mentioned, you need to be able to use this certificate for it to cover the needs of the piggyback FAA license.
2. You need to have a current FAA Flight Review, as that covers both the flying skills and the knowledge required. But I would hazard a guess that the intent here is to review the knowledge of FAA procedures, which would not be covered by the pre- and post-flight briefing for an EASA training flight. The best way to cover this would be to do your SEP revalidation with an EASA instructor who is also a FAA CFI, as this person would be able to make sure that the needs of the FAA Flight Review are met.
Just my two cents, I realise that this is a tricky subject...