Ian, I think it's good you try to bring up African bows. I have a thread on a Brazilian indian bow, but it hasn't garnered as much interest (if I remember right only you commented on it
). But that's okay, I think like you said, a lot has to do with the fact they aren't as easily preserved so antique bows are harder to come by...
Ian, the bow in the post#6 resembles somewhat the "gullwing" horse-bows of the Plains Indians in North America.
The D shape of the bow in Colin Henshaw's post is a good design in some ways. I'm glad Timo Nieminen cleared up its pros (has obvious cons). I'd like to add that some ancient Egyptian bows were shaped like this too. I think it is before they adopted the reflex(?)
I have noticed many African bows having a circular cross-sections. This is good for solving both arrow-paradox and stability in your bow-hand but there's some issues with it. Flat bows, and semi-circular bows like the English longbow, have their own set of pros and cons.
Some things off the top of my head...
Ethiopian/Nubian archers were extremely infamous for their skill in archery, and it was this martial tradition that kept all sorts of invaders out, from the Romans to the Muslims. The Ethiopian archer's were also known for being able to snipe the eyes of opponents wearing armor and helmets. Even in the time of the ancient Egyptians, Nubians were considered some of their best archers. I think that's one area to look if you are looking for powerful African bows.
The Hadza are known for their strong longbows. They loose with the "mediterranean" release and have an interesting forward leaning shooting form. Some of their bows have up to 100 pounds of pull. Their way of life, culture, and they themselves are endangered.
Many have observed African archers stringing their bows by stepping down on the belly of the bow (side that faces you as you shoot) while one end is on the ground, pulling the top end towards yourself and stringing it. Apparently they do this in Papua New Guinea as well.
Kenyans were using bows in political-tribal conflicts pretty recently. Not sure if you can get a-hold of those but they might be the most recent form of African martial archery.
Another good place to look is, as mentioned before, West Africa. Here's a photo of 2 Congolese warriors (not sure which ethnic group). Both bear bows. As I understand it, large hosts of warriors used to be mustered by leaders, and they mostly used a combination of spear-and-shield and bows, as well as blades and throwing knives secondarily. I have heard that in the Congo region, it is known that some will use leaves to make the fletchings out of... and that in some areas, they do not use fletchings. Fletch-less arrows are also found in Papua New Guinea and Taiwan. Fletch-less arrows are fine at closer range, and in Papua New Guinea their loooong fletch-less arrows have long heavy heads to keep the forward tilt and prevent the arrow from going nose up mid-flight.