Although there is KIND OF a postage system in my husband's country, it's really sucky. The way they do addresses is pretty much like a description of where his house kind-of is, if that makes sense. For example, when I asked him his address for the I129F he told me "Colonia 1ro entrada principal 75 vrs al sur" translated says basically "the first primary entrace in the colonia area, 75 varas to the south". You would think that because it's not a normal way of writing an address there would have been issues but there wasn't.
Based on my experience, USCIS isn't going to be terribly picky with the fiancé's address; Especially if as you say there is not an existing postal system he certainly won't get anything in the mail. Just as long as YOU the US based fiancé is able to receive mail, that is what is important
Is there anything like that in Equatorial Guinea? More like a description of where they live as opposed to an actual address?
I hope this helps or at least gives you less worry about your situation.
