I can confirm that BA-MRU is receving approval for the outbound and inbound passes through LHR (one a stopover and the other a transit) as that is what I have ticketed from them and began with flying MRU-LHR last week, with the final segment ticketed as LHR-MRU. I too wonder if this will continue to be allowed when the Comair MRU-JNB begins operation. To avoid any subsequent problem with reissues (which I will need to do in the spring), I plan to have it done by BA.