This is pre-1900 but doable: US does 1924-type quotas in say 1832. South Africa might be no Brazil/Argentina/Canada who get most of the immigrants but it gets quite a few, especially anglos. You end up with a big South Africa covering OTL's SA, namibia, zimbabwe, botswana, lesotho, swaziland. It's around half white, has an averaged out standard of living comparable to OTL Italy -- whites/coloreds live like people in advanced bits of the first world while blacks live at roughtly OTL Puerto Rican standards of living. Outvoted afrikaners mean Rhodesia-type qualified franchise is the norm until the 1970s when blacks get the vote.