Absolutely not, unless Comair is also serving the 3 gateway cities (and I suppose they must be serving at least one). The 2nd entry into Africa is only allowed to the 3 specified cities, so you cannot fly LHR-JNB or LHR-CPT (or MAD-JNB, etc) if you originate in MRU! Very important and inconvenient rule (unless this rule has been changed in the past week).

The return to Africa has to be LHR-ACC/whatever you want to do in Africa, probably surface segment to JNB. Likely to be a somewhat expensive one-way ticket (or a long and gruesome land journey).

edited to add that I almost always go eastbound (it is so much more efficient), thus I cannot do HKG-JNB-MRU or SYD-JNB-MRU but have to re-enter from Europe. For westbound the new Comair service does solve the MRU problem nicely, but can it be booked yet?