It's not precisely on topic, but you might be interested in Jacobus Capitein. He was born in Ghana, became a slave and was subsequently freed and educated at a European college, with the goal of becoming a Christian minister. In 1742 he gave a dissertation (in Latin) at the University of Leiden on the topic of whether Christian liberty and slavery were incompatible. He argued that they were not.

This wasn't exactly a defense of slavery, though. At the time, arguing against slavery was probably never going to fly. The thesis was largely concerned with arguing that it was appropriate for slaves to become Christians and remain slaves (thus ensuring that slave-owners did not have a vested interest in preventing conversion).