That unfortunately won't work: in JNB your luggage will be offloaded since you need physically carry your own luggage through the customs clearance area before dropping it off again immediately after the customs lane (customs situated between baggage carousels and re-check/drop off area). This is based on customs clearance regulations.
Re-check for SA Express and SA Airlink is actually even more inconvenient since they do not have a re-check facility directly after the customs clearance lane: in stead you would need to go to the normal luggage drop off in the main check in area in terminal A (domestic terminal), in the same place as the "online check in drop off" is. The crux being that at baggage re-check your luggage will be weighed again, irrespective of how or where your luggage came from. Here off course you won't have *G benefits, meaning potential luggage overweight problems.
Sorry for the bad news
