I Thought

[Contingency]

You could play as the papal states?

[PerXX]

It's possible, if you're willing to edit some files.

Have a look HERE.

Be aware that it'll probably not be all that much fun.
Every Catholic faction you attack are automaticly excommunicated. You won't be able to call crusades at will either.

[Contingency]

well i tried what was suggested in that thread but moving the papal states to the list of playable factions on that note document didnt seem to work?

[princeps.omnis]

Hotseat campaign!

Start one, choose any faction (say England), then in the console type "control england" to put england in ai control and than type "control papal_states" to control the... papal states!