Dave Fowler gives good advice about cast iron rod or brazing.  I've had
piano plates done both ways, but the real trick is in stress relieving
the casting before and after the fix is completed.

One thing that's not been addressed through all this is if the plate
was installed improperly at the get-go.  Everywhere the plate touches
the wood behind it, it should do so with pretty much equal force.  If
there's a gap or gaps in places and screws/bolts are tightened such
that the plate is stressed in the process, then additional stress added
from string tension, the results could be disastrous.

I still strongly recommend, should one decide to have a plate repaired,
is to use someone very familiar with repairing engine heads.  Discuss
with them the problems, and what the plate has to do when functioning
in the piano.

Some of the naysayers for welding _may_ have had a bad experience.
There are so many factors involved, that knowing which plates are
candidates for repair and which aren't takes a seasoned technician to
decide, and even then, bad results are bound to happen.

As it is, I have a lovely Bechstein planter out in my yard: a 1917 6'2"
grand, the plate from which was badly repaired three times prior to my
getting the piano.  I decided to take the piano out of circulation --
one whack with a 4-pound sledge and it broke in three more places.
Rest in pieces!

Tom Binnall