i once saw an accustic banjo that was amplified using a standard less paul humbucking pickup mounted between the rods inside of an epiphone banjo. along with a thin (about 2cm) piece of steel placed directly under the bridge.
i'm not sure why it works(the whole phisical vibration into an electical signal gets me)but it does work. the tone was very electric but still banjo and the signal was a little weak(hey, but so are fishman).
definetly not for traditionalists but IMHO an interesting apraoch.