patowalker
I fear that you are mis-interpreting that letter, which is relatively well known in air-law circles.
Your original 'grey area' relates to somebody operating an N-reg, with both a UK PPL, and a US part 61.75 license based on that UK licence. They are free to do that anywhere in the world.
The letter refers to a different situation - namely, a UK pilot flying an N-reg aircraft elsewhere EASA land, with his UK-issued EASA licence. No 61.75 piggyback licence. Here he is attempting to place reliance on FAR 61.63(1)(v)
The FAA opinion is that even an EASA licence has a country of issue (such as the UK), and that whilst this will allow such a licence to be used to fly an N-reg in the UK (in this example), it would not allow the pilot to fly the aircraft in another EASA European country. For that, he would need, either:
1) an FAA licence (61.75 or native); or
2) another EASA licence issued in the country in which he is now flying.
I hope this helps.