I don't mean to pop your idea balloon but there are many flaws with this idea. (Assuming my knowledge about the MTW2 Papal system is correct.)

1. We could only do this for one faction, since there is only one pope.
2. There would be no way to restrict it, as the game function is meant to help you conquer the holy lands easier. Changing it to restrict your armies would be unrealistic and seemingly impossible.
3. This would only allow you to attack someone else once every few years.
4. The pope is supposed to be invincible, if he dies someone else takes his spot. If his only city is taken he becomes a horde. Doing this means the senate would forever live, even if Romani was destroyed.
5. If every army was a crusade/jihad army then the people would desert if the army sat around for awhile.