and access to either some very enthusiastic amateur smelters, one enthusiastic professional smelter, or some really good welders, because wood you can easily find, but a pre-cast plate for an unusual size piano... very not so much.
I'd think that welding would be the sensible route if you're just doing one, as long as it's made sturdy enough. Casting the whole frame in one piece makes more sense if you're mass-producing pianos. Also, it's traditional. I don't know what welding equipment was like a hundred plus years ago during the piano-manufacturing boom, but casting piano frames was a well-honed art.
Better be really well-engineered as the the string tensions on a conventional grand piano ranges into the low 10s of tons. Yamaha pianos, for example, are around 20 tons.
