I can't see a Juche-like apartheid South Africa, but what I can see is an actively expansionist apartheid South Africa attempting to create puppet states around it, especially if they feel under threat after Rhodesia becomes Zimbabwe. Probably wouldn't last too long before the regime crumbles due to its own internal strife, however.
Juche would be alien to the mindset of Afrikaners or other white South Africans. It's a borderline religious doctrine which demands absolute faith in the "Dear Leader" who is incorruptible and can't be criticised--that contradicts Christian religious doctrine. It also requires the whole nation to be united behind the idea of self-reliance, and if these are anything like the 20th century white South Africans we know, that definition excludes the majority of the population.
Not neccesarily disagreeing with you, but before the DPRK, Pyongyang was known as a major hub of Christianity in East Asia, to the point it was nicknamed the "Jerusalem of the East"
Juche is thought to have borrowed from Christian symbolism and rhetoric to some extent accordingly, and people have drawn parallels between Kim Il-sung and Jong-il and the Father and the Son.