I agree that mercury is generally not something that you want in your
environment, although using mercury as a switch is not a bad idea in
itself.  A mercury based switch will arc like any other mechanical
switch, but because it is a liquid it will not get contact burn; as
such, a mercury switch should last almost forever.  Also by its very
nature a mercury based switch has a very soft touch which is probably
why they used it in this case.

On the environmental side of things, with how much current the motor
takes, each time the switch turns on or off the arc vaporizes a small
amount of mercury into the air.  This most likely will be absorbed
by the wood of the piano and probably not much will get out of the
confines of the case, but none-the-less it _is_ getting into the
environment.

I tend to agree that modern technology (with perhaps the exception
of a MIDI circuit board) has no place in a classic player.  Although
I think depending on the type of restoration or "build up" some modern
materials are _sometimes_ appropriate (see my earlier MMD postings on
"Automotive hose and copper pipe" and all the controversy that caused!)
Even though your LM331 circuit does work, sometimes simplicity is better.

What I would have done to get around the mercury problem is to build
a small "period looking" wooden box with a hole in the top for the rod.
This box would then houses a modern "soft touch" mechanical switch.
The rod would then push down on the switch.  This would replace the
mercury cup and still look like it belongs.

Musically,

Ray Finch
Albuquerque, New Mexico, USA