Please can you point me to the regulation that states that you need a Biennial flight check, in the USA, even with a CAA/EASA piggy back licence?
FAR 61.56.
FAR Part 61 Sec. 61.56 effective as of 11/15/2013
The BFR does not need to be accomplished in the USA, nor even in a N reg aircraft, but must be with a US CFI.