Hello to Jurgen South Africa!! One of our favorite places in the world--and one of the hardest to nonrev....
Sadly, there are issues beyond seat count that can effect your chances. One is station mgr doesn't want to bother w/nonrevs if it means a delayed takeoff. You have to get cleared (after all the paperwork--cargo etc) then get thru security and make it to the gate.
HK's are inconvenienced OR unassigned seats for rev ticket pax, and rarely, Pos Space staff.
What date? Maybe I can give a second opinion. We travel S2 thru JNB and once didn't make it and had to connect thru Dubai.