Reading the PTS for the Instrument Rating, it has words such as "applicable navigation systems to include GPS, FMS, VOR". So the question becomes what do they mean by "applicable". I suspect, that means asking about the four-leg-range, LORAN, or navigation by bonfires would not be applicable. However, just because it isn't on the aircraft that is being used that day doesn't mean it cannot be an oral question. Extrapolating that logic, I would agree with what ryan1234 wrote.
On a multi-engine check, it would be quite fair to ask an applicant what the advantage of counter-rotating propellers is and why does it work that way or what is meant by the "critical engine" regardless of what kind of airplane was brought to the test.
However, as a matter of practice, the examiners don't venture far away from the aircraft you are using for the test.