Thanks for the answers. Interesting to know that AC and UA are exceptions to the rule, but sadly no help for me this time.
The question is whether I'm prepared to drive 3-4 hours more each way just to benefit from the miles, lounge etc of a flight to/from JNB. Could be interesting, though, to experience one of the more obscure European widebody flights into a small airport.
(Namibia is, it bears repeating, a STUNNING destination for a holiday. Went there in 2019, with KLM via Luanda, then onwards with now-defunct Air Namibia to JNB. If you are reading this and you ever get the chance to go, don't let the lack of EB-earning stop you.)
Dunes at sunrise, on the road to Sossusvlei, sunrise (2019).