Piezo pickups can often be found in "normal" store bought six string acoustic guitars. They are more susceptible to feed back in high gain environments such as the stage than a electric guitar. They have been used successfully on stage by thousands of musicians over the years so they can defiantly do the job if treated in the correct way
You can easily and cheaply purchase some piezo disks and a jack connectors and fashion you own pick up and it can produce some very good results.Resulting in a guitar that will act very similar way to a normal acoustic guitar when amped. More likely to feed back than an electric on stage but still very possible when approached in the correct way. They can also be used for recording http://www.youtube.com/watch?v=3LzNnatRBIY This track uses two guitars fitted with home piezo pickups
There are a few things too think about, firstly a piezo disk of approximate diameter of 20mm will have a quite a high output, certainly more that a electric pick up. Also they will have a higher output impedance than an electric pick up.
In most "normal' acoustic guitars that have a pickup there will also be a small amplifier installed this helps to resolve the impedance matching problem by acting as a buffer between the pickup and amp. Most cigar box guitars do not have a pe-amp installed so we can suffer from a high output and impedance mismatch which will also contribute to feed back.
That combined with pick up placement can create a real hot rod of a guitar ready to disintegrate in a explosion of feedback when unleashed on an amplifier
After trying a few different ways of soldering the pickup in I came the conclusion wiring in parallel improves the impedance matching greatly but output is still quite hot. I have found covering the ceramic of the one of the pick ups with hot glue improves the tone. The piezo covered in hot glues will provide a more bassy tone and the other one will be more toppy. Together they make a better all round fuller sound.
Finally the placement must be considered after trying a few places I would say the nearer you can get to the bridge the better. Ideally right under it or on the opposite side as show above. I have found the signal to be too much if the pickups are mounted to the actual guitar top but you may have more Joy.
Good luck !
You can buy my Guitars on line http://www.ecrater.co.uk/p/13233791/cigar-box-guitar