Apart from experiments into laminated Balsa, I have not strayed from Spruce for soundboard bracing. For back and side bracing however, I have tried a few different woods that were stiff/strong enough, quartersawn and not too heavy. It appears that Baltic Birch is a general classification, and can vary greatly from piece to piece. If you cut a brace-sized stick from your stock and test it, it may prove to be suitable for back bracing.