The Comair JNB-MRU flights (Wed, Sat, Sun) exist on BA.com, Amadeus, Worldspan and Sabre. The return MRU-JNB flights exist only on Sabre. According to BA, it's the Roach Motel of the Indian Ocean. (Old US advert - roaches check in but they don't check out.) There would soon be an interesting collection of Comair 737s at MRU. Obviously this will be rectified in due course.

Good advice has been given. Work with CX. If they can see MRU-JNB in their system, get it ticketed but open-dated, and wait until you're in Africa to add the "real" dates. If CX in BKK can't help, they can send it to HKG where surely someone will be able to sort it out, even - gasp - BA??