I have a rough and ready solution. Looking at the Ancillary Triggers we have this.
Code:
;------------------------------------------
Trigger trigger_patriarch_1
WhenToTest CharacterTurnEnd
Condition EndedInSettlement
and RemainingMPPercentage = 100
and Trait Christian >= 1
and SettlementBuildingExists >= church_christianity_basilica
and IsGeneral
and SettlementBuildingExists >= monastary
and FactionType empire_east
and Trait Christian >= 1
and not FactionwideAncillaryExists patriarch_1
and SettlementName Alexandria
and not Trait ChristianDisillusion >= 1
AcquireAncillary patriarch_1 chance 16
Ergo the super easy shortcut. I'm not 100% sure if this trigger is applicable towards Traits bit it would be a PITA if it wasn't.
There is another way. Just create a unique building that is present in the settlement from the beginning of the game. And then run a trait trigger through that. IE
Code:
;------------------------------------------
Trigger zoroastrians_hermitage
WhenToTest CharacterTurnEnd
Condition Trait Zoroastrian >= 1
and EndedInSettlement
and RemainingMPPercentage = 100
and SettlementBuildingExists >= hermitage: Replace with the newly created buildings name.
Affects HatesChristianity 1 Chance 4
Affects ZoroastrianLoatheChristians 1 Chance 1
Affects ZoroastrianDisillusion 1 Chance 1