http://www.caa.co.uk/docs/33/CAP804Jan2013.pdf
Found it, and actually have not done it myself - thanks for pointing it out!
The thought occurred that if the PIC is signing off on the aircraft and him being the sole manipulator of the controls, the stateside guy would get his advantage out of his local regulation and you still are PIC. Now, trick question: There was a flight that was legal in a SEP in the States, an accident occurs, who is PIC? By CAA account, do they toss a coin? Or is the safety pilot PIC for air-to-air collision, while the guy flying would be responsible e.g. for an empty tank?
The interpretation of the CAA does not convince me from a legal point of view (and since not two EASA-country flyboys share the cockpit) - but why risk arguing with them.
As for the practical value: Sharing the cockpit under the hood is MCC at its earliest and best, but obviously this is only about legal logging of flight time. My 40 hrs of solo flying from Wichita to LA and back taught me a lot, so there's merit to both schools of thought. Anyhow, commercial (under part 61) in the States is 250 hrs vs. EASA land is 200. So that evens it out ;-).