Not just militant nationalists, but you could see concerned people drawn to the plight of ordinary South Africans head over to fight. Similar things have happened in the Middle East. In OTL you had aid workers from places like the U.S. in South Africa helping out, some were even killed during the violent period of the early '90s.Given that White nationalists from the west may be attracted to the war in South Africa, why not Black nationalists from the United States?