It's pretty frustrating. Only Papal holdings seem to count toward occupation war score. The only way to win is to simply wait until you start getting time based war score which will give you 100% after many, many years of sitting around on top of an occupied Europe that's trapped in a war that can't be properly fought because the Pope can't be attacked directly when he is landless. Crusades should definitely be disabled in the Pope is landless, IMO. If you want a crusade, you'll be generous and donate a Bishopric.