South Africa has had expansion on its mind since its founding. See Ronald Hyam's book on the subject: 'The Failure of South African Expansion, 1908-1948.'
For example, Southern Rhodesia had a referendum in 1922 about joining the Union, but it failed. Some suggest that it actually would have helped Smuts retain power and lessen the influence of the National Party.
That there is the key. Britain would never turn over colonies with large African populations to a blatantly racist regime. This is particularly the case regarding the High Commission Territories and one of the reasons why they were never given to South Africa for administration. This is also one of the reasons why any talk of South African expansion ends after 1948, the National Party is now in power and starts to really implement apartheid.
Also, note that the National party did not really want more Africans in their country.
Possibly, and I am very hesitant to say that, possibly there could be some sort of union in the 1970s with Rhodesia and Portuguese Mozambique, but I can't see how the international community would allow that. Furthermore, the South Africans didn't want to bring pressures down on themselves, hence why they stopped support for Rhodesia.
Basically, unless South Africa is run by a United Party that eventually allows majority rule, they aren't going to be given any territory. If they want to take land, they will most likely be crushed.