If the Portuguese manage to effectively occupy Central Africa, even if only with trade posts, small forts, etc., the Brits won't challenge their dominion over the land. Brazilians could definitely help to occupy the land, don't expect mass migration as Brazil has plenty of empty land, but some adventurers and freedman could help Portuguese colonial effort. IOTL a considerable number of Brazilian freedman migrated to Togo and Nigeria, these people could be directed to Portuguese Africa even without a more elaborated scheme similar to Liberia.