While I doubt it would ever be encouraged, and the results would vary from key to key, it can be done with some pretty amazing results. Check out Walter Moutons B flat.
Randy Falcon took a set of C reeds and filed them down to B flat. I figured it might have just been a one hit wonder but then when I asked Randy about it he pulled out his very own personal B flat with a set of C reeds filed down to B flat. Sounded just as good, if not better, than Walters.