The best way to get this is by establishing a Portuguese colony in the Cape during the early 1500s. Early Portuguese colonies suffered from severe gender imbalance, mostly due to sailor superstitions saying that women in ships caused bad luck. Miscegenation through intermarriage between Portuguese men and Xhosa women would start immediately. After that, it's only a matter of time before the favorable conditions of the Cape cause the mixed-raced population to explode. Then, it can spread througout a large part of southern African, perhaps in a way that is analogous to the Brazillian
bandeirantes movement.
Regarding the enactment of policies deliberately promoting miscegenation, Iberian and Latian American nations were inconsistant about it, but there exist case where those policies were, in fact, enacted. More competent colonial administrators in Portuguese Asia, such as Afonso de Albuquerque, encouraged their men to settle permanently in India and Southeast Asia and take local wives. These were called
casados (married men), and colonial administrations frequently gave them land confiscated from Muslims.
I don't know if there were any laws, but there was definitely some form of racialist pro-miscegenation movement in Brazil during the 19th century.
This painting is called
The Redemption of Cain, and it shows a black woman thanking God for the birth of a white grandson (the woman holding the baby is her mixed-raced daughter).
Also, Paraguay went as far as to ban the marriage between people of the same race during the de Francia dictatorship.
I'm not trying to propagate the myth that there was no racism in Iberian cultures, I'm just saying that, given the right circunstances, state-sponsored miscegenation can, indeed, occur.