Really? I didn't know that. I imagine, though, that they were just preaching and not and looking to establish a colony for freed slaves.
How likely would you say that Liberia could've been in Namibia, or even Gabon like you mentioned?
You are quite correct on your assumption, if I recall correctly.
And it really is a matter of preparation and investment. To even reach Namibia, and bypass the other regions, you'd need preplanned intelligence saying that it's good... And, being on the edge of a great desert may hardly be attractive for immigrants, as there is no potential to their life aside from scraping out a living on the coast. Of course, greater intelligence may show how inhospitable
From, say, an identical situation in comparison in OTL? I'd rate it as very unlikely. If the ACS was larger/better funded/had government sponsorship, you could see scouting, or even claiming a geographically significant area. Southern Angola would be the "best" open area, but actually getting surveyors inland to try and find a location would be quite difficult.
With enough support, most locations in Western Africa mentioned are... possible, at the least. Liberia, Gabon, Namibia... even South Angola, or even the Congo are possible, but the last one would mean going up against the Kingdom of Kongo and Portugal as well.