It's an interesting idea - and I have played with using mirrors. But I run into a few problems:
Most mirrors will actually show a faint double image due to the silver being behind the glass. This becomes more pronounced if you're shooting at a steep angle. Single surface mirrors are pricy, and finding one that could slip through the f-holes of a violin is a challenge.
Also - if you're outside and pointing in, you're running into some similar issues - the f-holes are simply too thin to telephoto through without them blocking image at least partially - and then you're no longer shooting wide, so you wont see much of the instrument in the reflection anyway.
