I asked on reddit/askhistorians if there was a serious effort by Muslims in East Africa to promote Islam in Central Africa, besides the historically known fact of making raids into Central Africa for slaves, up into the 19th Century. I am assuming that the answer will be "there was no effort to spread Islam into Central Africa from the East."
So I will ask the what if aspect of my question. What if Eastern African Muslims DID make a serious effort to spread Islam into Central Africa? For one thing, it would have delegitimized King Leopold II's claims of fighting slavery in Congo to help establish his genocidal corporation regime known as the Congo Free State.
I would argue that with a unifying, and possibly militant ideology such as Islam, uniting the Congolese and Central African tribes, they would have been more inclined to resist, and resist more effectively, European expansionism. Additionally, they could have received support from what Islamic kingdoms existed in Africa during the mid to late 19th Century if European expansionism was more aggressive in the face of stiffened Congolese resistance. Given the extreme climate of the region, I do not think the Europeans would have pressed further in the region if the Congolese tribes did put up a serious fight.
The Islamic nations that existed in Africa during the 19th Century may have also seen more profit potential in trading rubber instead of slaves. The question is, would Islamic traders treat the Congolese as basically slaves, using Leopold II tactics, or would they have traded with the Congolese tribal people as business equals? In the later case, we could have seen a very different economic development in Congo. I would also think that the East African Muslims, though perhaps more adaptable to the climate extremes of Central Africa, would have thought it was not worth it to subjugate the Congolese if they really did put up resistance, due to climate conditions.
Brian Ghilliotti