The following two clips shows one of the most complex pup configs I've ever seen, including mag and piezo.
Sorry I am a month late to this discission (newbie!)
I came across this via another post and just wanted to add that I have now completed 3 CBGs with electric PUPs and piezo disc configuration. I also read it would not work and so for the first one fed the input via a takamine preamp I picked up off ebay but then realised it sounded better when played on the preamp bypass circuit so for my last 2 CBGs have wired a typical telecaster type wiring with a 3 way switch, normal PUP and three piezo discs wired together as one pickup for wiring purposes so the options are electric pup/both/piezos and it works great. It offers 3 distinct tones. I am unable to explain why it works but don't agree that it acts by shorting because both the electric PUP and the piezos both work on a tap test when both are selected.
I think for any more CBGs I make I will include this set up as it is easy to do and I like the mild chorus like effect you get when both are played together.