Have seen the self-same fog phenomena on a few occasions at BRS myself. No fog on the rwy at all but only over the transmissometers. Much discussion as to what to do with the approach on the first occasion, with the older and more experienced captain in the RHS urging me to land and ATC implying that it was below minima....managed to persuade the ATCO, in the end, that his transmissometer needed looking at prior to the approach ban point so he declared it u/s and we landed with everybody happy. It is a stupid situation though and one where common sense should prevail. A case of where 'rules are for the guidance of wise men and the obedience of fools....'