FWIW I was unable to book CPT/JNB-LHR on my DONE5 even waitlisting several months (admittedly I was locked into specific date). OW Emerald counted for nothing, but maybe high status on BA would have released a seat (there was plenty of full fare seats available just zeroed in D)?
I ended up routing to LHR through SIN (since I was coming from SYD).