The Dutch of the Cape Colony was de facto polygynists, what we need are that it's made de jure before the Voortrekkers migrate. Maybe VOC see a benefit in increasing the local labour pool by legalising and it's integrated into the local culture. Honestly the result will not be more White South Afrians, but a larger multiracial Afrikaan ethnicity develop, where Coloured will be seen as a integrated part of it. A much large Afrikaans speaking population will likely push new immigrants to adopt Afrikaans rather than English. So the result could be that English end up only spoken by a small minority (but still being prestigeous think OTL Malta), while Afrikaans push the local native African language out too.