Thanks for the reply... seems extra baggage will be surprisingly affordable.
Before you'd replied, I called the SAA call center to ask them, and was told something slightly different, namely, that ZAR 300 is the charge for domestic baggage, and ZAR 450 is the charge for international flights. Since the agent didn't seem to distinguish between international destinations, nor did she seem interested in what the website said, I gathered her advice could be disregarded.
For future reference of anyone interested, I'll report back on what's actually charged....