Glen, I really don't know. But my instinct tells me that SAA would instruct agents not to through check luggage if the connecting time is under the MCT. Whether they would be pedantic about 10 minutes probably only actual experience will tell.
However, at that time of day the international terminal at JNB is usually very quiet (unlike the evening stampede!). So I would actually rate your chances of making the onward flight very highly even if you have to re-check your luggage in JNB. With OLCI and access to J check-in facilities this should be doable with normal ops.