I mean Liberia but as large an powerful as OTL South Africa.
Best I can tell, just off the top of my head, is either a region between the Niger and Ogooue Rivers (if I recall correctly, Spain had a claim in that exact region 200 miles inland, but they did not defend it at all; Equatorial Guinea is the result) or somewhere further south. Both are farther from the US, though, so I'm not sure how you could get there early on (an earlier Spanish-American war or something? The Rio de la Plata maintains control of that claim and gets in a war/sells it to the US? Just thoughts).
The biggest problem in any part of that region is that the American/European populations can stand the conditions of Africa that well, along with the natural environment, makes it very inconducive to settlement. The southern end of Gabon has a better climate than Liberia, certainly, but it doesn't hold a candle to the temperate zones of southern Africa, nor those in Kenya, Ethiopia, Madagascar, etc.
To have a South Africa-like scheme occur in a Liberia alternate, you need to have a large, settled population grow rapidly throughout the 19th century so that they can be a larger component (if I recall correctly, Amero-liberians were 0.5% of the population; compare that to about 10% for the Boers). You may not need that many to establish strict minority rule, especially if there is a coalition of the privileged vs the remainder.
All that said, I could see a Liberia within the area of Gabon+Republic of Congo+Equatorial Guinea still being successful, as Liberia had one of the highest human development ratings in Africa before their civil war. Be interesting if alt!Liberia became a destination from some spaceflight companies due to their equatorial location.