I haven't done this so take it with a grain of salt, but Stewart MacDonald sells a Five-Star brand magnetic pickup that mounts on the rim coordinating rods and has a thin steel shim placed under the bridge. I don't know if it would be possible to mount it on a single rod or dowel stick as most open back banjos have, but I'll bet it could be done with some modification. They claim the pickup provides natural acoustic tone. They recommend a pre-amp for stage use. The price in the latest catalog is $82.90

Jon W.