Angola is a corrupt, bureaucratic nightmare. It is not the kind of place I'd be jumpseating thru to try to save a few bucks. Even the flights from Luanda to JNB are limited (Angola only allows SAA a certain amount of frequencies) so you could easily be stuck in a craphole terminal for an undetermined amount of time, and getting into town for accomodations is challenging, at best. And I hope you have the right visa.
I know World Airways does some charter flights for European operators and they are in JNB regularly.