I, too, have had several of these pianolas through my hands.  Rewiring
has always been done carefully.  I have never received a shock from
a switch.  I always earthed the switch and motor housing like David
Evans.  Once, though, after one of the machines had been playing for
10 minutes or so, I noticed a very hot smell.  I found that the motor
casing was too hot to touch.  Obviously a short.  Removing the earth
line to the motor casing solved that.  The leak detectable was a matter
of a volt or two, and didn't cause the house mains breakers to trip.

Rather than sell this one on, because of a potential safety hazard to
a customer, I kept it.  It is still performing beautifully 15 years
later.  I have tied a warning notice to the motor casing, for those who
come after me.  They can have it re-wound!

Best wishes,
Nigel Perry