About a decade ago, I constructed a self-playing guitar using some of
the parts made by Ragtime West.  Ragtime West's Ken Caulkins will sell
any of the components; buying some of them saved me lots of work.  An
entire "picker" unit can be had for about $1300.  Information about
parts and pricing are in the Ragtime catalogue, which is on-line at
http://ragtimewest.com/Catalog.pdf 

Ken designed and patented an interesting picker unit.  Eight pickers
are mounted on an octagonal, ratcheted wheel.  Each stroke of a
pneumatic pulls the wheel one pluck via a lever and small mechanical
overriding clutch.  There is a wheel for each string and the whole
assembly snaps together in a universal frame that can be used to pluck
any number of strings on any handheld stringed instrument.

The frets are simply dampers attached to pneumatics through brass rods.
His pneumatics are plastic units with integral valves and mount on
a square or round bar.  It's the same stuff he uses for pianos --
pretty universal.  Then there are MIDI valve attachments you can buy
or you can use a roll frame of your choice.  I made some modifications
to the design, but overall Ken's ideas are good, and his parts are
somewhat dependable and adaptable.

The Ragtime West components are vastly different from the wooden
hand-made parts with which most of us are familiar, but their design is
clever and they are easy to use and they last fairly well.  Compared to
a pneumatic made by hand of wood and cloth, they are probably
relatively affordable.

Good luck with your banjo project.  If I can be of any assistance,
don't hesitate to write.

Ed Gloeggler
Long Beach, New York