This can't be settled without reading the entire FAR book and working out the interaction of every clause in there with every other one, and probably never with 100% certainty.
However, if you turn to page 159 in the 2005 FAR/AIM, Subpart A - General, $91.1 Applicability (a)
it states ...........
"within the United States, including the waters within 3nm of the US coast."
This "USA only" qualification extends, as far as I can see, all the way to Subpart H - Foreign Aircraft etc (page 211 of FAR/AIM) where it says
91.703 (a) (2) ... when within a foreign country, comply with the regs relating to the flight ... there in force
and
91.703 (a) (3)
... comply with this part so far as it is not inconsistent with applicable regulations of the foreign country where the aircraft is operated or annex 2 of ICAO...
The bit which one could have fun debating is 91.703 (a) (3) where we could argue the extent to which a regulation prohibiting something in US airspace is applicable to a different country's airspace where the same thing is not prohibited (though not expressly permitted).
It is a fact that N-reg places can legally fly in UK airspace, and it is a fact that the UK has a different airspace structure. The privileges of the IMC Rating are what they are because of our airspace structure - one can see for example why the rest of Europe doesn't have an IMCR also.