I've had excellent results welding gray cast iron using pure nickel
wire.  I have made several successful repairs of engine blocks and
manifolds.  Nickel has a coefficient of thermal expansion close to that
of the iron.  I have never repaired a piano plate, but I'd certainly
try it.  Pre/post heating is not usually necessary, but the normal V
grinding is important.  You might inquire at your local welding shop.

Good luck,
Jack Hardman