I did the trip you are planning in the Fall of 2000. Obtained Congo (Brazzaville) visa in Yaounde (Cameroon), and the Angola visa in Libreville (Gabon). From Libreville you can ship bike by air to Johannesburg by twice-weekly flight (Air Gabon). You can go south to Pointe-Noire overland, and cross over to Cabinda (which I did). In Cabinda you can find a ship to take you to Luanda. If the situation warrants you can try to go overland to Namibia. If not (in my case), you can find a ship to take you down to Namibe. From Namibe I was able to ride to Namibia via Lubango. The ships are irregular, but at least at the rate of one a week.Cheaper than flying from Libreville, and certainly more adventure.