For what it is worth, here is my experience of this issue.
I have a JAR PPL. Based on that I went to the United States in July 2007, complied with the TSA and FAA requirements, was issued an FAA PPL and then trained for and passed my American instrument rating. This resulted in the issue of an FAA licence including the endorsement "instrument airplane" but did not include the endorsement of English proficiency nor a statement to the effect that I had passed my USA test. (I do not believe that this last point makes any difference but it was mentioned on two separate occasions by the people in America to whom I spoke about this that this endorsement was also worth having).
When I became aware of the requirement for the English proficiency endorsement, I contacted by telephone and e-mail:
. AOPA UK. Result: they e-mailed AOPA USA who seemed not to know the answer
. AOPA USA directly by telephone who suggested I contact the FAA direct
. The FSDO in New York and the FAA in Oklahoma City
. UK and German FAA international field offices
All to try to find a way to avoid having to make a special journey to the United States to obtain the English proficiency endorsement. I made all the obvious suggestions such as visiting an international field office or the United States Embassy in London.
My contact at the New York FSDO, who was very sympathetic to the absurdity of having to make the special journey, also spoke at length himself with the FAA in Oklahoma City. The contact at the FAA was also sympathetic and spent a considerable amount of time researching the issue.
The conclusion of all of this, which took about three weeks, is that I was advised by all of these contacts either that, at least at the time when I spoke to them, they did not know the answer (AOPA) or, if they professed to do so (FAA & FSDO), that there was no alternative to visiting an FSDO in person.
Part of the reason for this is that the international field offices both in the UK and other European countries deal exclusively with technical issues and do not handle licensing questions. As indicated above, I have telephoned the UK and German offices and got the same answer from both.
As a result of this, I ended up flying to Las Vegas in March to have the endorsement put on my licence (I chose Las Vegas because the flight training organisation where I did my instrument rating has a full motion simulator there and I thought I might as well do some emergencies training and thus get some sort of value out of the trip).
The inspector at the FSDO in Vegas could not have been more friendly and helpful and was genuinely sorry that the system had conspired to force me to make a special journey explaining that the reason was not down to the FAA but the Transportation Security Agency (TSA). He also offered, unprompted, to add the endorsement that I had passed my US test. We even shared a good laugh about an Englishman proving to an American that he speaks English when he, the inspector, contrived to write "English Profficiency" [sic] on the top of my form!
At the end of the day, I now have a temporary certificate showing the English-speaking and US test passed endorsements and am awaiting the return of my plastic licence. The need to do this in this way was, to say the least, a monumental pain and very expensive in terms of time, money and jetlag but at least I got to spend a day in the simulator which was extremely useful and taught me a great deal.
I would be very interested to hear if anyone has actually been able successfully to follow the route of obtaining the endorsement from one of the two UK inspectors mentioned in earlier posts. Certainly, the information I was given was that you could not do this but, at the time I saw the posts, I did not want to contradict what seemed like authoritative advice given that I had already made my arrangements to travel.
I hope this is useful.

, I has now received the following e-mail from the FAA in New York: