The Pope is made playable with most of the MTW mods out there, I can haertily recomend MTW XL, by Vikinghorde, you can find links in the MTW mod forum.

@EYG
I've only done it once out of masochism really but I did pretty much what you say. I didn't make any military moves nearly, lots of agents, especially assasins to take other factions out.
There were no stability issues, the XL mod has GA goals for all factions, though most are just the conquest and homelands ones, I think the others are hard coded.
Obviously I couldn't excomm or order crusades when I was playing, but in campaigns where the pope is an AI faction, all these issues work normally, he'll call for Crusades, give out Florins to good conquering Christians etc!