Here is a nifty preamp circuit that provides a FET buffer and balances the signal, which deals with all the issues you are discussing. The circuit is based on the work of Alex Rice.) You attach the piezo to the circuit with a length of shielded mic cable, then run a mic cable from the circuit to a male XLR. This plugs into your recorder which supplies ...
Have you looked into condenser microphones? Instead of the dynamic mics, condensers are used for more intimate sound recordings. Additional, the Sanken brand makes some microphones for such needs. "Sanken [makes a mic] called the MO-64, which is capable of recording the heartbeat of a snail."
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You will need a wireless lav mic.
Two manufacturers of such systems at entry level prices (as wireless can be very expensive) are Sennheiser and Shure (there are other manufacturers, of course).
Now, one issue is to find the way to hide the mic under your Mark Twain clothes. You might need some training, find appropriate accessories (like gaffer tape or ...
That's a great question. I think your explanation of the problem would be good to share with customers, educating them rather than providing them with a meaningless spec. Then maybe link to a couple of sample recordings of common sources recorded with your mic attached to some likely surfaces.
It really is an interesting problem. Wonder what Shure, Crown, ...
I can vouch for JrF: http://hydrophones.blogspot.se/2011/05/c-series-pro-contact-microphones-new.html
I never leave home without them. Using an H4n.
They are used by BBC, which according to me is a good reference.
Jez riley French is an established name in hydrophones and also contact mics, especially for people who find Trance's offering "on the expensive side".
Also see this: Can you suggest a good contact mic for sound design? (average cost of proposal would be much appreciated)