No. You have to be de Jure I think.
Could you destroy your king title and revert to your ducal title? Might be worth a try.
Only way I have survived in vanilla is with an Axum Shia ruler via the Beard Designer and swearing fealty (a Nubian Shia still doesn't work as you just get hit with conquest DoWs), it's easy enough to expand that way, and eventually the Fatimids start to break up once the first Caliph dies if you help them on their way a little with some discontent and helping factions.
With the mods and so on, it's possible, but still pretty hard. The biggest issue after a while if you haven't been able to expand northwards is that all your provinces are poor 3-4 slot at most.
I've given up trying it as although I can see it's posible, it's frustrating and down to too much luck in the end, and I can't be bothered reloading all the time when something happens as it spoils the fun. I'll hopefully wait and see what will happen in the next patch IF it gets some attention. As it is, everyone seems more keen for extra boobs in-game rather than balancing and bug-fixing, oh well...