Try Lufthansa. Business class availability on SEA-FRA has been pretty good over the past year or so. Then they do FRA-JNB on their own metal, codesharing with SAA for the final JNB-CPT leg, though I have no idea what award inventory is like on FRA-JNB. If you can't do an award all the way it might be worth it to redeem for SEA-FRA-JNB and just buy the last hop on Comair or SAA.

Another option would be to depart from YVR on KLM and try YVR-AMS-JNB on KL metal all the way. This would allow you to dodge the awful DL SEA-AMS service where properly priced award redemption is nonexistent.

If you have any OneWorld miles you might also try YVR-HKG-JNB on CX, going the other way around, as it were.

BA is ridiculously expensive with the dreaded YQ so I think LH and KL ex-YVR are your best bets.