Making it special to specific settlements is an option, maybe just messages? Perhaps along the lines of 'like minded people met in cologne and invented fish & chips'
Never say no to an easter egg....
As for characters in same field army - if I'm correct DLV mod army upkeep script could match it.
The idea is to measure general_unit attribute percentage and grant trait ArmySize. If it's 100% we have single general unit in stack, Trait ArmySize=1, and so on.
If I remember well, second named character in stack always had ArmySize 1 level. So maybe this is the way to solve it with some necessary changes.