I thought I had covered all options (except ID90, etc.), and doublechecked all carriers with gates at JNB. All except DL and LH were codeshares without zed agreements. Then I checked other African cities and the only other option below Morocco/Egypt seemed to be LX to Kenya. Then, inspired by your question, I started checking the route maps of every "partner" and (alphabetically the last to check, of course) happily found that VS appears to have a flight from London... so there is another option. Thanks again!