A while back I became aware of a product called "Lake One Wood
Repairer", available from Garrett Wade (and others) that has become a
staple in our piano repair shop.  It is a two part system, consisting
of a powder and a liquid hardener, and, frankly, it is amazing stuff.

Here's how it works...

Mix the powder with the hardener in a disposable cup, like a wax or
plastic cup, using a popsicle stick or tongue depresser.  You don't
want to mix too much at one time, to be explained later.  Add hardener
to create the consistency you need:  runny to fill cracks and screw
holes, pastey to fill chips or missing sections of wood, or doughy to
form new features.  I saw a demonstration where a carved music desk had
an "ear" broken off, and replaced with an equivalent sculpted piece of
LakeOne, and it was as strong or stronger than the original wood, with
total adhesion.

Anyway, to fix a screw hole, just mix the right amount of repairer, and
drizzle it into the hole, making sure to pop any bubbles, and then fill
again so that it is slightly proud.  Then, wait a few minutes (no more
than 5) and it becomes leather hard.  This is your time to take a sharp
chisel and slice off any proud material flush with the original
surface.  Then, about 15 minutes later, it will be fully hard.  Drill
an interference hole (the diameter of the screw shank) and then screw
in your screw.  That's it!  You'll NEVER strip out that screw, period.
I even saw a demo where a screw had been coated with grease and mushed
into fresh, uncured, LakeOne.  When the material had cured, the screw
could be removed just fine, but if you tried to overtighten the screw,
the head broke off.  It's that strong.

The reason you don't want to mix up too much, is that it cures really
fast, and you'll never reach the 20'th screw hole before it becomes
unworkable.

When cured, it can be stained, sanded, planed, finished, you name it.
It's REALLY HARD, you can't dig a fingernail into it.

I had an old upright with what looked like a shark's bite out of the
vertical corner of the case.  Two minutes with Lake One and I'd created
a new corner, stained and ready for finish.  You basically can't see
the repair if you work carefully.

I don't sell the stuff, just use it.  I and several of my fellow
rebuilders rely on it daily for everything from action to case repair,
and its speed makes it perfect for in-line use.  There's no wood filler
or epoxy that can touch it.

Neil