download the oneworld tripplanner from CX, for your planning.
it has all oneworld schedules, including code-shares

a quick answer is: no, without detour or supplementary 1-way tickets

LHR-MRU, okay, but you can't get abck to africa continent from MRU
(there is flight from SEZ_NBO though)

so this is what you could do
(all on BA and its franchises, so perhaps you could also seek advice from the BA people there)

lhr-jnb-hre-nbo-sez//mru-lhr
you need to find a way to get from sez to mru yourself -- not sure if there is a cruise, that would be perfect