Sorry, I mis-typed. I meant to write Broadway, not 6av. The original post has been edited to fix the error.
Worst case scenario, you'll end up with one or two B's trapped north of Prospect Park that for whatever reason have to be turned at MetroTech. It's not the end of the world. With the W terminating at Whitehall as it is, it doesn't matter WHAT you send up Montague (B, D, etc.); if you want to turn it before 34th st, you have to turn it at MetroTech anyway.
Given how rarely such a scenario would occur now that SAS is open, a marginal analysis would show that it would not pose too much of an operational problem to assign R32s/R42s to the B over the long term.