Presumably the OP already has a second OW carrier (as it was a OW award to begin with, there must have been at least BA + another)...

As for the stopover question: CX flights to JNB generally leave late in the evening, allowing easy <24-hour connections from either SFO inbound.

Availability, well, that likely will be a problem... but stranger things have happened, and if you don't ask, you don't get. (although, two J-seats to JNB is asking an awful lot)