Just for the odd occasions when the rest of the Hounds allow me to use my banjo (4 string) I use a soultion similar to Richard's with a mini humbucker mounted at the end of the neck, (although discreetly under the skin) and a passive transducer mounted on the skin, just under the bridge. The two are wired together to a single output, and the blend seems to work without the need for a blend pot, although I usually run it through an EQ pedal.

The only visible sign is a jack socket in the rim, and the acoustic sound is unaffected.

John