I think the main problem with attracting a large number of white immigrants to Africa was that you already have a large pool of unskilled labour that places like Canada, Australia and to a lesser extent the United States lacked. There was a large out migration from Southern Europe in the 1950-1973 period, however most of the emigrants went to other European countries, mainly France, West Germany, Switzerland and the Benelux countries and the United Kingdom. These countries were close meaning that transportation was cheap and often men would emigrate alone leaving their families behind and often it was temporary. Those that chose North America or Australia were able to find unskilled jobs paying much higher than those in South Africa or Rhodesia. In fact some jobs payed so little that workers were recruited in Mozambique and Malawi.
The immigrants South Africa and the Rhodesias attracted during this period were overwhelmingly English-speaking for a reason. Most immigrants went to work in the service industry or in the civil service, most also had some sort of capital and were educated. The few that did go into the agricultural sector were generally well off as farming in Africa was very capital intensive. There were some immigrants from Southern Europe including Portuguese, Greeks and Italians. They generally dedicated themselves to commercial ventures, owning restaurants, small markets or being middlemen.
What the South African government needed and wanted were doctors, engineers, technicians etc. If white rule had lasted longer I'm sure that the government would have attempted to attract Eastern Europeans in the 1990s. After the collapse of communism, the former Soviet Bloc had a large pool of highly-skilled individuals that would have been highly desirable. Being that the SADF strove for self-sufficiency due to international sanctions and I'm sure they would have tried to attract Russian or Ukrainian personnel from the former Soviet defence industries.
If South Africa manged to become home to a large enough Russian diaspora of say 100,000 or more, this could impact Russo-South African relations. The Russian government may try to be soft on South Africa to protect the large Russian community. As far fetched as this may seem, Greece and Portugal were often reluctant to be too harsh on South Africa in the 1980s for that very reason.