I don't mean to offend anybody by what sounds like a "quick fix", but
the more I work on old stuff the more I find that using stuff that
works and is fast is better than spending twice as long with something
that is "ethical".  I therefore am a big advocate of cutting off the
ends of cable ties and using those in the screw holes.  They work
great.  By cable ties I mean those little plastic fasteners that lock
-- see http://www.1stcableties.com/images/cables_pic.gif

I picked up that trick from a pipe organ shop I work in.  Theater
organs are of course much bigger than player pianos, with lots more
screws, and are a lot of work to get going.  If we spent all day trying
to repair stripped screw holes by putting in wooden plugs or other
things, the organs would never get playing.

I've tried Plastic Wood, I've tried wooden toothpicks, I've tried
carpenter's glue...  I'm telling you, plastic cable ties work the best,
they're cheap, and they're fast.  Besides, the screw holes are stripped
already, you're not making things worse.  You don't glue them in, you
just snip off the end and drop it in the hole.  They work like toothpicks
are supposed to, but they don't disintegrate like toothpicks do.

Damon Atchison
http://www.msu.edu/~atchiso5