Er, it does if you wait for the Umayyads to become Hispania, though it might be difficult since the AI doesn't seem to form titular kingdoms to cheese the requirements. You'll be a vassal king of the Empire of Hispania. I think to pay fealty to the Carolingians these days you have to border them (in which case you probably don't need to pay fealty to them, since that means you've managed to take Navarra and take the fight to the Umayyads or the Carolingians have formed Francia or the HRE and they have the Umayyads against the ropes) so you're much better served allying with them instead. ETA: Converting to Sunni won't put you out of harm's way. The Muslims get plenty of CBs against other Muslims, though it will make you less of an inviting target when the Umayyads need to win holy wars to avoid a decadence revolt.