Herodotus' Histories, Book 4, Chapter 42
My emphasis in bold.
"Libya is washed on all sides by the sea except where it joins Asia, as was first demonstrated, so far as our knowledge goes, by the Egyptian king Necho, who, after calling off the construction of the canal between the Nile and the Arabian Gulf, sent out a fleet manned by a Phoenician crew with orders to sail west about and return to Egypt and the Mediterranean by way of the Straits of Gibraltar. The Phoenicians sailed from the Arabian Gulf into the southern ocean,
and every autumn put in at some convenient spot on the Libyan coast, sowed a patch of ground, and waited for next year's harvest. Then, having got in their grain, they put to sea again, and after two full years rounded the
Pillars of Heracles in the course of the third, and returned to Egypt. These men made a statement which I do not myself believe, though others may, to the effect that as they sailed on a westerly course round the southern end of Libya, they had the sun on their right - to northward of them. This is how Libya was first discovered by sea."
This gives us the earliest reasonable time for the tech transfer. Necho II ruled from 610 to 595 BC, and hired Phoenicians to circumnavigate Africa.
This should just predate Bantu invasion of South Africa. Have the experience rub off on the local khoikhoi (the cattle herders) who would very naturally trade beef to the fleet. Alternatively or additionally:
A. Marroon at least some part of the fleet in S.A.
B. Have some part of the fleet decide they want to settle.
We could handwave horses in- probably a population that will suffer for being too inbred, but far, far superior to nothing.
The resultant people: Genetically, almost completely khoikhoi. Culturally, more than a bit Phoenician. Only get some essentials (iron, agriculture, horses, other domesticates, weaving). Population density should skyrocket, causing all kinds of societal turmoil, reorganization and advancement.
When the Bantu show up without an agricultural package adapted for the climate, they're just going to bounce off.