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I am not sure: but can't you create triggers when you can join... and making one of those trigger an event having happened or a certain trait that can just happen on certain conditions?
 
This is a trading society mod that allows you to build warehouses and sell specific goods or use them to add bonuses to your counties you either do it yourself or assign someone to be a trader for you it adds a lot to the game mostly the trade republics or people with the buisness focus join but trading goes on a lot in westeros too. Adds a whole new reason for you to interact with the free cities and beyond.

https://steamcommunity.com/sharedfiles/filedetails/?id=920653668

Its compatiable with the mod so maybe you could contact the author to get it put in or just try it out and take some ideas from it.