Unfortunately SAA tends to reserve their upgrade space for their Voyager members only IME. Also their business awards are difficult to obtain on their IAD-JNB flights.

OT, but I would suggest going through Europe and spending a few hours in LHR or FRA in a transit hotel, lounges or exploring. I do the trip 1-2 a year and found I enjoy the Europe stop more than the non-stop. You can upgrade on UA to Europe and earn the bonus RDMs and BIS for lifetime/E-500/CR-1/SWU as well. The SAA Y product is nice on their 346 but I still like the break. PM me if you would like advice on specific routing and times...