The welding of cast iron is very much an art, and there are people
who have surprising skill at it.  They're not easy to find, and some
persistence is necessary: you basically have to lug your afflicted
plate from one welding or machining outfit to the next, searching out
the grizzled old craftsman from amongst the stacked engine blocks and
bulldozer blades at each stop.  Then, without appearing to apply any
pressure, you show him the thing and ask if there's any hope.
Sometimes there is.

Note that there is no requirement that the welder know anything about
pianos, and in fact it might be best if he didn't.  What he'll have
spent most of his career repairing are cracked irreplaceable automotive
and industrial parts.  If you see a shop that has a lot of old 1956
Mercury exhaust manifolds in for repair, that's a good place to try.

I learned about this when we had an antique 'street clock' to repair
at the science museum where I often work.  It is a huge cast-iron
affair that stood on the sidewalk outside of a jewelry store in
Columbus, Ohio, for seventy-five years.  The thin cast-iron shell was
cracked in several places, partially because it was carelessly handled
and partially because it was the worst piece of casting ever performed
in North America.  The boss trotted a piece of the thing to maybe four
places before he found an outfit that said they'd try to fix it, and
they did.  Wasn't cheap, but it's all intact now, and on display in the
museum's upstairs hallway.

Good luck with it.
Mark Kinsler