Hi JAC,
The only things like that which i recall are the hand
held meters that estate agent surveyors used to carry
to test for damp.
They have two little spikes which get jabbed into the
area in question and a reading is taken from passing
current between them.
Using this on wood may not give a measure of moisture
more a reading of surface conductivity.
To get some idea of the moisture content i would suggest
a miniature version of a metal detector, this would still
respond to surface conductivity, but also to internal
conductivity within its range.
Maybe use of both methods would give some reliable
indication after a bit of practice.
Cheers, John