You might ask the administrative officer of the US Mission to Geneva (or at the Embassy in Bern, but it's smaller) what the experience of American FSOs assigned to Switzerland has been. It's true that they are relieved of certain requirements (just as foreign diplomats can import for the duration of their stay -- just as foreign tourists can -- unapproved and uncertified models of cars to the USA).

But the Mission is likely to have standard advice they give out on the subject, and perhaps the names of garages that undertake conversion work.

When I was assigned to Geneva in the early 1990s I didn't bother registering my car locally. I took advantage of my status to keep a UK-registered car, and the only downside was I got only temporary passes for tax-free petrol.