Probably similar to parts of Brazil. Large mixed-race population, lots and lots of slavery by means of an African version of the bandeirantes who are nominally out trying to find gold.
I wouldn't be so certain that a Spanish South Africa would have a small population, either, since South Africa is well-known for gold and diamonds, both of which had a tendency to attract Spaniards in the colonial era. Since you'll be having all this gold/diamond mining going on, Spain will definitely fight to keep the place more than they would in other circumstances. Plus it'll have a decently large colonial population of its own.
The issue, of course, is the Tordesilhas Line which grants South Africa to Portugal.