A few more thoughts on stripped screws.  First, it is difficult to
imagine suitable places to use drywall screws in a player piano.
They have huge heads and are either black oxide finished or zinc
plated -- in other words, really ugly.

Wood screws with tapered threaded portions are a really terrible idea.
It's difficult to imagine why they persisted so long.  They require
tapered pilot holes or at least a different hole for the threaded
portion and the shank or they bind or strip, etc.

Sheet metal screws are a good alternate.  They have straight threaded
portions and hold much better without the need for a special drill bit
for the pilot hole.  You can often replace a wood screw with a sheet
metal screw and get a decent hold without having to reinforce the
original hole.  You can get them in stainless steel that looks a lot
like nickel, and you can also get them with slotted heads that look
more original.

Plugging holes with dowels is almost useless if you use regular
dowels and do not drill a cross hole to insert them into.  This is
not practical in many cases.  You can get or make cross grain plugs
which can be glued into a hole made in the direction of the screw.
If you only need a few you can use the cheap plug cutters from Harbor
Freight.  If you need to make hard wood plugs or many of them you will
need a better quality cutter.

Sticking toothpicks in the hole actually makes the situation worse,
in the long run.  They do not hold well because of the grain going the
wrong way and they also tend to enlarge the original hole making it
worse.

A good temporary fix for larger screws is to cut a length of nylon
"zip tie" and stick it in the hole.  You can use two or three pieces
if necessary.  Be sure to cut the material short enough that it does
not interfere with the head of the screw, especially if it is a flat
head.  The nylon has just the right properties to hold the screw well
and to bite into the original damaged hole pretty well.  There might
be some suitable material in smaller sizes for small holes but I have
never tried to fix them this way.

Spencer Chase
Garberville, California