Well, I couldn't find this command either, but I found this in "events":
Code:
Identifier: GiveSettlement
Event: A settlement has been given to another faction
Exports: faction, settlement, region_id, target_faction, target_religion
Class: ET_GIVE_SETTLEMENT
Author: Lee
Although it's not a condition, the description does give that impression... I don't know, maybe someone could enlighten us.
edit: Basically it says that when a settlement has been given to another faction blah blah blah, so it won't help you. It may be impossible to issue such a command. But you could try to give a diplomatic stance between the local faction and the papal states a -1.0 every turn, perhaps that could do the trick.