OK so I have partially answered my own question:
I can transfer Amex Reward Points to Starwood at 3:1 ratio, then Starwood to Marriott at 1:3 ratio, then Marriott to SAA Voyager miles at this crazy ratio from the SAA site:
Voyager Miles Marriott Points
1 500 Miles 10 000 Points
3 500 Miles 20 000 Points
7 000 Miles 30 000 Points
17 000 Miles 70 000 Points
35 000 Miles 140 000 Points
At the top conversion, this seems to work out at buying 1 SAA Voyager Mile for 4 Amex Reward Points. Seems kind of expensive, especially as I need 18 750 Voyager Miles for a one way flight from TNR to JNB. I'm giving up 75,000 Amex points to save about 430USD. Not a good exchange rate!
Anyone have any better suggestions for earning SAA Voyager Miles? Maybe a cunning way of earning Marriott Reward Points? Or SPG points?