Hey El, Zappa used to have multiple piezo PUs in his guitar neck, so people have played with the idea (no pun intended LoL) so it's not such a crazy idea. The piezo is going to PU/respond to vibrations in the wood around it, so it won't act like a conventional electric guitar PU. But if you want to get some of the vibration going through the wood it will pick that up. So where do you put them (I've not heard of just using 1 in an electric guitar), Zappa chose the neck (exactly where
). E.g. You could put one in the peghead (like clip on tuners use). To put one in the middle PU position, you want to know that there is a good sound in the wood at that point/spot of the body.
Piezo PUs first came to light for acoustic guitars and you drilled a hole in the bridge and epoxied them in (anyone remember Hot Dots). That picked up the vibrations in the bridge. So they are a very different transducer (PU) than the electric (magnetic) guitar PUs we are used to which cause a signal to be induced in the coil from a revolving/vibrating metal (alloy) string.