I've been a registered piano technician for 15 years and have rebuilt
many pianos.  Is your piano repairable?  Possibly.  There is a cold
repair method (non-welding) that works very well known as "metal
stitching".  The company you want to check out is "Lock N' Stitch"
http://www.locknstitch.com/ 

I've done it, it works.  Generally speaking this assumes that it is
a simple break.  In your case, however, with breaks in four different
places, you are rowing upstream in a leaky boat.  I wouldn't even
attempt to repair one in that kind of shape.

Welding cast Iron is very difficult.  It's high carbon content makes it
expand dramatically when heated and when it cools the stress is only
transferred to a different spot, encouraging it to break again, and
believe me they do.

There is one reliable means to weld a plate.  This involves heating the
entire plate evenly until it glows red in a special oven designed for
this purpose.  Then it can be welded hot so it won't expand in the
process.  The same company mentioned above performs this service but it
is very expensive.

On the other hand, if you should find another _exact_ plate from
another piano then that might be the best approach.  Remember, however,
that _something_ caused it to break -- it didn't break by itself.  If
someone dropped it during moving that's one thing, but if the cabinet
and back posts are warping and have distributed the tension some place
it shouldn't be, that's entirely another story.  First find out what
caused it.

Rob Goodale
Las Vegas, Nevada