I stumbled across the VIAS Violin Analyse System which is along the lines of
the cennecting a transducer to a vilion/cello thread from a couple of weeks
ago. Link is below. On page 7 of their pdf manual they show a thin steel tube
running between a magnet/transducer and it rests up against the bridge, kind of
like a violin string. They seem to be using this as measurement input into
their system, but it looks like a similar method could be used to vibrate the
bridge. Maybe that's what they are doing to generate a signal into the violin.
I don't know. Just including this for completeness.
http://iwk.mdw.ac.at/VIAS/englisch/vias_features.htm-Elhardt