A US (FAA) Captain of an N-reg Boeing can fly wherever he wants, just as I (FAA PPL) can fly wherever I want with an N-reg. The real question is: can I (FAA PPL) fly in Spain or France with a G-reg a/c? Let's say the US (FAA) Capt is flying a D-aircraft over France. Would that be tolerated?
It is up to the UK CAA to decide what privileges, if any, the holder of another ICAO license has in a G-reg, and this is the case no matter where the G-reg goes. This is no different to him having a UK license, once the ICAO license has been validated by the UK CAA.
It is up to the "German CAA" to decide what privileges, if any, the holder of another ICAO license has in a D-reg, and this is the case no matter where the D-reg goes. This is no different to him having a German license, once the ICAO license has been validated by the German CAA.
Etc. That's my understanding, anyway.
As for France: I know a Dutch Pilot who flies a F-reg a/c with a FAA PPL. This is according to what is mentioned earlier: you can fly with a FAA PPL on a F-reg plane if you're not a resident and for VFR only.
That is probably in accordance with what I heard previously, for what that is worth.
The thing is that I recently wrote the French about this but they said I had to convert FAA -> JAA. I wrote back and told them what I am writing here, but all I got is silence since then
That doesn't mean anything - unlike the UK CAA, the DGAC is terribly disorganised. Myself and others have spent months chasing after a person capable of generating a simple piece of paper for a Socata aircraft. For example I am sure a lot of people in the DGAC take one look at a communication and if it is not in French they bin it. You've got to be a fluent French speaker and get on the phone to them and escalate the matter to the top. Their switchboard is French only and all the time one calls them in English, even if asking for a person by name, they tend to hang up (my experience from 2005).