Seems to me there ought to be some East African groups that would be both matrilineal and Moslem. A roommate of mine in college was Shone, and they are matrilineal, and some of them are Moslem. I don't know if they give it up when they convert ~
There are the Luguru in Tanzania, but I am no expert on the success of Islam at assimilating African tribes. That is, I know the Luguru are traditionally matrilineal; I do not know to what extent they are considered orthodox Muslims. They might be politically dominated by Muslims, or in some stage of assimilation, or both.