Found this while browsing flights today:
https://www.google.com/flights/#sear...L941;a=SKYTEAM
Per KLM it books into L/V class which earns 100% qualifying milage on DL.
Same day turn may not be possible but I would argue that if you're flying down to JNB you could easily spend a couple days checking out its rich history and Jazz scene.
Safe Travels,
James