I have no idea if this would work, but to simulate slavery maybe you could put an african CIV with an unreachable capital (surrounded by impassable LM terrain) in the middle of africa, give that capital a great wonder that produces a worker unit every few turns; I don't know if the computer knows how to trade for workers, but at least a human player would be able to get at the workers on the trading table. You could also set Barbarian villages with workers in them in areas along the African coast.
The up side would be that Europeans would have to buy slaves from 'African Warlords' which are sent to their capital in Europe (a good portion of slaves were prisoners of war sold to the Europeans) and other slaves could be acquired by raiding 'small African villages' IE the barb camps, and then transport them to the New World if they wanted to use them there. At least, it makes more sense to me.
Of course, that might not work at all, too, and I really doubt the AI would be able to use it to full effect. The idea might have been mentioned somewhere else, for that matter.