The license is on the person not the plane. I have renewed my IMC rating in an N reg before but that was in the UK. I just dont know if it is one of those funny things where by the retest can only be done in the UK.
You are correct in one instance that you cannot use the IMC abroad for flying in IMC conditions but it does removes the restriction that I always have to be in sight of the ground hence I can fly VMC on top (i.e in France)....which is just what is needed sometimes.