As a violin player as well as harpsichord player/maker I never heard of
anybody using violin peg soap on a harpsichord. For violins it’s for
making the tuning smoother when the pegs stick, due to inevitable wear
or bad taper fitting and mostly because of weather conditions, etc. A
harpsichord tuning pin will turn nicely and stay put if it fits the hole
properly. No need for this.
As an aside, peg soap has been made by various manufacturers since the
19c but the recipes are very closely guarded trade secrets. I read that
only two people at Hills knew the formula. Nobody really knows what is
in them. There’s quite a lot of discussion about this mystery on violin
forums. Most likely they are a type of rosin combined with chalk. It’s
very interesting stuff because it has to perform two contradictory
functions at once - allow the peg to turn easily, and yet then make it
grip tight when in position. I find it fascinating. I use the Götz
Wirbelsiefe.