I don't think they need to add any bonues per say, the bonus is already there - you collect a large bounty of trade income. They just need to make the AI understand how important it is and be willing to dow for it if they must.
I think it's rather the opposite. The AI colonises too much of Africa (and I suspect getting a presence in multiple trade nodes is already responsible for 'patchy' colonisation; Portugal or Spain jumping to Chesapeake, then west and southern Africa rather than finishing painting initial colonisation areas).
A better representation of what jockedahl and I are talking about would be significant bonuses to colonial growth elsewhere, for colonising (or being 'given') a single province in any given African region, representing a slave-trading post (and a naval staging point towards the east). A deal with the neighbouring African nation, even a protectorate, simulating them providing slaves in return for arms.