That power was the power of excommunication, which basicly all catholic nations had execute by removing the excommunicated leader.
You can have all SA realms include that in there oath/treaty to Corsanctum.
It will give corsanctum and the church lots of power.
Well if ALL the priests of SA were in Corsanctum, and if they exerted their power via auto da fe and religious take overs against the theocracy they want to put in line, perhaps it could work.
It's still not very easy to realize anyway; the main achievement would be having nobles more "loyal" to the church than to the state, to the point that a ruler that upsets the church seriously risks to lose his power to an inner rebellion based on religious reasons. That was more or less something Morek went near to a couple of times, but still not many are ready to acknowledge this special status of Corsanctum within SA. Perhaps thas has more to do with Mathurin's desire not to impose himself too much more than game mechanics however.
