Very very unlikely... there are about a dozen flights a day on BA on this JNB-CPT route and when I checked minutes before calling AM, there were 4+ seats in J and 4+ seats in Y for redemption. I still suspect entire flight's redemption is put on hold as long as there is a seat that's been reserved for a OW redemption (not sure whether the fact that this is a Comair flight operated as BA has anything to do with it).

Will check as soon as my AM redemption is ticketed to see if the flight hold is released.