The grand duke is bang on. What I do to open up an enemy to jihads is make a large roving unit of troops under a good general and jump from enemy province to enemy province. I beat the enemy or force them to withdraw to their keep or otherwise abandon the province, but I don't siege or hold anything, instead assaulting the neighboring enemy province the following year. Then when they reclaim what is theirs, it's jihad time. I usually drop 3 jihads on the reclaimed province adding any princes I have for the piety bonus.