Surely if the bike has passed the "loudness" test in its country of registration be it the UK, France, Italy etc and it is Euro compliant and you are passing through then the cops cant stop you as you are not breaking their law as it is not registered in "their" country.
I would take a copy of the MOT/ Registration with you as the Swiss cops love reams of paperwork.
I would tell them to sling it in a polite way and if they have a problem get them to contact DVLA via the phone