Thiril: here's how we did it:
1. Pick a random zealot girl (but even normal will likely do), marry her to a landless person with a claim to caliphate (or even a heir maybe).
2. Invite her to your court (this is where zealot plays: she hates her liege and will easily accept). Her husband moves in with her.
3. Pay him, ask him to convert, kill her, marry him materlinearly to someone of your dynasty, have them bear kids.
4. Force claim (or kill Caliph if your guy is a heir).
5. Kill new caliph, have the kid inherit, make the kid your heir, kill yourself, have the kid inherit. Kid will be a catholic caliph.
This allows to call Christian crusades, not Jihads. You have new CBs when declaring wars - Crusade for <kingdom>. The message about how pope has issued a crusade will pop up. And I only did this on 1.05, no idea about 1.06.