I have currently been researching exactly the same question.
And from all the answers I have come to understand that the road is mostly paved from Morocco to South Africa except for a section between Gabon and Congo. Just make sure you are crossing that section during the dry season and you should be fine.
We're planning the trip at the start of febuary next years so we will hit the rainy season!
What are you planing on driving?
My main interest at the moment is in exploring the possibility of Ceuta to maybe Accra on tarmac. Ten week cycle down to Accra and ten weeks back up to Ceuta. This is where it gets complicated....... using a agricultural type tractor pulling two trailers........as in a tourist train. I need to find a way of working the train from end of Sept to end of March.