Sorry to be the bearer of "bad news" but I'm 99% certain that those coached passengers will still be counted as BRS passengers and not as BHX passengers as the CAA normally allocates passengers on diverted services back to the airport which they should have flown to/from.
Mind you, if they can combine BHX and BRS passengers on to a single BHX service, it does suggest that one airport or the other must have some quite low loads on BACon services?