Back to missing Rs. I have noticed in some non-rhotic dialects that Rs in places where it is very difficult to lose them tend get transmogrified into Vs. Specifically I am thinking of the local pronunciation of Cleary Square in Boston, which is pronounced something like Klee-uh-vy Skway-uh. Perhaps something like this is happening to brought. The resulting V could easily get assimilated into the B.

The fact that you know when people are pronouncing brought "wrong" means that no lack of understanding is resulting from the production of the new pair of homophones.