In my opinion piezoelectric is not a good choice for capturing and amplification the acoustic violin sound, if that's what you want. The best way is a combination of contact microphone and small unidirectional microphone, like a Schertler STAT-V.
A guitar has a lot more wood surface so a hole drilled for a jack isn't going to hurt the sound. Obviously a fiddle has less wood to work with and is more delicate, therefore it will be more susceptible to changes. Anything that hampers or interferes with the vibration will affect or mute the tone. I'm no luthier though. And there a plenty of companies out there making pickups that work very well on violins, some more permanently installed and some not.
I made an experiment to see what happens when adding some extra mass to the ribs. I took a table knife and wedged it in behind the chinrest holder. When I played a string with the bow, the knife vibrated, which indicates that the ribs do have some sound. I wedged it in a little harder until it appeared to be mute, and I experienced that I had a slight loss of tone. The volume became a bit weaker and the tone a slight bit shorter.
However: I did the same thing with a teaspoon and got the opposite result! Still the difference was barely noticeable, but to me it sounded as if I got some more sound. Longer tone and louder output.
Volume, if that's all your after is easy without drilling into the violin. I have used both custom bridges with integral rare earth pickups and the wedge style where a small wedge pickup is attached to the bridge. Both are more than loud enough. What happens to that vibration after you have captured it is the most important i.e. preamps,EQs and amplifiers. I live by the standard that I only am custodian of the instrument for the time I own it. I do not ever want to do something to alter the original structure of the maker. Even cheap violins are really incredible feats of engineering.
Svento Rivers said:
I'm still interested if such thing would affect the output volume. If it's possible to mount the jack in the ribs without ruining the sound, I'd definitly prefer that solution. I wonder because on acoustic guitars such things doesn't affect the sound, but I've never tried an electro acoustic fiddle, so I don't know what the acoustic output is like. They're generally piezo-amplified too, so I guess the presence of the pu itself kills some of the sound.