Crow Custom Cast Welding in Hudson, Wisconsin, repairs cast iron.
I had him do work on a stove for me and he usually does a lot of
pre-WW2 car manifolds and engine block repairs.  He and his wife are
very nice folks.  His web site is http://www.castmetalwelding.com/ 

It's not an inexpensive process, but he heats the iron up to a "dull
black" and then makes the repair while it's hot.  It takes a week or
ten days to cool it back to room temperature, and then it seems like
he puts it through one or two more heat cycles like that to relax the
repair.

I don't know if he would take a piano plate on, but it wouldn't hurt
to ask.  He will certainly give you some ideas on the thing if nothing
else.

He spent a whole afternoon showing my brother and I the work he does.
Most of his work gets shipped to him so he doesn't see too many actual
customers at his place!  He showed us a semi-trailer full of pre-1920
engine blocks waiting for repair, a Cord engine he had repaired and was
waiting to get shipped back, and a V-12 Cadillac or Packard engine and
manifold that I would have written off as scrap that he was working on.
He's an artist, and when he gets done and it's sandblasted, you won't
see the repair.

Well, good luck with your quest.  It really is hard to know who to turn
to in a deal like this, but I really doubt these guys will take the job
on if they don't think they can do it.  And I really respect that.

Earl Hennagir