I know very little about pianos, and I am perhaps the nation's
worst welder.  But I know some of the best: they post their stuff at
rec.crafts.metalworking and are eminently qualified to discuss the
welding of cast iron and essentially anything else.  Some of them could
cast you a new plate in their own home foundries, or mill one out on
one of their computer-controlled milling machines.

There was a short discussion about piano plates some years ago on
rec.crafts.metalworking. I believe that one of the more metallurgical
types told us that piano plates varied wildly as to the quality of the
alloys therein.  Some were excellent gray iron and others fell short of
the quality lavished upon iron window sash weights and barbells.

In any case, the various builders of miniature steam locomotives,
racing and aircraft engines, and spacecraft components should have some
good answers for you.  There is also a book on the welding of cast iron
that was sold by Lindsay Publications, and they may still have it.

Mark Kinsler
http://www.frognet.net/~kinsler