Its hardly ASB, if South America was colonised by Europeans then there's no reason Africa can't be. Even southern parts of the US could be considered largely inhospitable, and yet were still conquered. The same goes for areas of jungle in the East Indies, where there are still rumoured to be cannibal tribes and undiscovered regions. And thats a lot further from Europe than Africa is. Besides the only region of Africa that was never a European colony at one time (and for a long time, in Angolas case in the 70's!) is Ethiopia. The key is having one power rule over it, displace tribes (I admit the larger population of African tribes is a stumbling block to this ATL, and yet colonisation and segregation comparable to US/ Native American relations was still 'achieved' by South Africa and Rhodesia) and then all you need is a certain discontent with the rule of that foreign power (as happened for instance in South Africa and Rhodesia whilst under British rule). Its not even neccessary for America not to be discovered, as others have demonstrated. It might be ASB territory to suggest a reversed Atalntic slave trade for instance, given the less suitable conditions for crop cultivation in Africa, I'm not suggesing an exact USA replica, I'm more interested in the differences if anything.
That would be a problem no doubt! However do you think its possible if the early immigrants settled in North Africa, and within a few generations became more acclimatised, then something like the 'gold rush' triggers a huge influx of population, it might be possible? I mean many Europeans died in the Americas too, particularly South America, but a continuous influx (bolstered by the slave trade admittedly) meant a steady population growth.