Pinblock plugs do work well and there are piano repair supply houses
that sell them ready made. Here is an idea, however.
Loosen the coil around the pin, remove the pin and bush the pin block
hole with sandpaper (sand side toward the wood). Drive the pin back in
and reattach the coil. One layer of paper generally works pretty well
although I have used up to two layers. This sounds like a pretty
"funky" fix but I have had it work many times. After all, it is an
older piano -- what have you got to lose? It is cheap, quick and you
can always go the plug route if the sandpaper doesn't work.
Oh, by the way, cut the paper to be just a little shorter than the
depth of the hole and _drive_ the pin back in the block. Good Luck.