I think it would partially depend on when such a Union would occur. Remember - even the most hardened pro-Rhodesia crowd have to admit that Rhodesia was essentially a huge diamond-mining company, operating out of South Africa, until around 1920, and it was only WW2, the Cold War, and UDI, that really grounded a (previously rather ambiguous) national identity in the Colony/Nation. A lot of African colonies had commercial ventures intertwined so strongly with their government/governance that it becomes hard to separate the two.
I guess what I'm saying is that early/mid-Colonial Era Southern Africa would be a very different political landscape to late/Cold-War Era Southern Africa.