I actually booked the RTW today, but 3(!) UA agents were not sure if the MAN-JNB-FRA was permissible - I could hear them discussing it. Anyway, to save buying the higher mileage RTW ticket I decided to stick with the straightforward one. They did say that if my trip started in MAN then if the FRA-JNB-FRA trip was at the BEGINNING of the RTW, followed by Atlantic or Pacific route then it would be permissible. Alas, my timimg does not allow this.

One final point, as I live in the UK I wanted to start my trip in Germany to attract the significantly lower fare (F Class). After a false start the agent (female) at the UA call centre in Dublin was extremely helpful and patient. She fixed up all my itinerary and billing in DM. Full marks tp UA