ex-NBO RTW travel has got much more complicated since BA dropped regional routes such as JNB-NBO. Since you have to leave then later re-enter Africa from different continents (with the exception of Nigeria or Ghana originations) I see no way around having to add a seperately purhased JNB-NBO or vice versa ticket to complete the loop, or simply start in NBO but finish in JNB.

To validate possible RTW itineries, go to this great link:

http://jph.bytestacker.com/award.htm