Well,my idea for now is for garrison scripts.
I know that we can't disband a unit via script or ~ command.But we can script in a FM with any unit, so will become his bodyguard.
E.g.When you(playing as Romani)siege a capitol of Quarthadast,the script spawns an army of 10 very old FM with carthage_city_militia as their bodyguards.
They(FM) should have traits like"garrison commander",not allowing them to move.
Due to their old age they will die quickly WITH their bodyguards,so we don't need to disband them!