Although I'd expect less scrutiny ex-JNB on visas than from other points as it's regional and they are far more familiar with the real world operations in Moz. Far flung departure points however are more likely to just read Timatic line by line ( and just speaking from experience going to Africa on a single ticket from NRT many times may even not have any idea on the map where Mozambique is.) If Timatic doesn't say VOA is available then I personally wouldn't try it from distant locations, going with an overnight in JNB en route would be a far less risky way.

I know the OP was talking about going via JNB, but there was another poster (Leo123) who talked about arriving on QR and attempting to go back out on the same plane. Their experiences could be quite different.