You can do the same thing with navies. The Papal States usually has one navy roaming around the western Med, and if you catch it next to an enemy fleet you can drag them into the war and cause your opponent to be excommunicated at the same time.

Me, I allied with the Papal States, HRE, and Byzantine Empire at the start, and made a hard choice to keep my global reputation up by not exterminating or sacking any cities and releasing all prisoners without asking for ransom. My relations with all 3 have been solid. Milan was treacherous as usual, and I had to take down their Italian cities (they had taken Dijon in my game) and Sicily jumped in at a later time. I just waited for the inevitable war between the Byzantine Empire and Hungary to break out, got myself dragged into it on the Byzantine side, then took Constantinople from Hungary after they took it from the Byzantines. This might prove to be a trigger for war with my ally, though. The turn I took it, during the end turn cycle, I was attacked by a Byzantine army. The game crashed a few seconds later and I haven't gone back to it yet to see if it will repeat.