I have it in writing from the UK CAA that
as far as they are concerned the IMC Rating is valid in any aircraft registration. The ANO does not restrict it thus, so it is unrestricted.
However, what privileges a given license/rating gives you (if any) in a given aircraft reg is determined by the State of Registry of that aircraft.
So I wrote to the FAA asking them if the IMCR was OK in an N-reg and their reply was affirmative.
However, if somebody is just asking whether they can renew the IMCR in a Swiss-reg plane, in Swiss airspace, I can't see why not. They need a UK CAA IMCR examiner.
Who is going to be PIC is a fun question... in a G-reg, the instructor/examiner is AIUI always required to be PIC. In an N-reg, the instructor/examiner is rarely PIC. In a Swiss-reg, I have no idea. Historically, to sidestep these questions, all kinds of checkrides were done in a neighbouring country's airspace
It is also not inconceivable that UK CAA IMCR examiners have been issued a guidance note which contains something like the checkride must be done in a G-reg. I have never heard of that though. In the CAA-approved schools in the USA, JAA PPLs and IMC Ratings are trained and checked in N-reg planes.