The sympathy works roughly that way already. The peasants of your regions are unhappy if you are allied with people they don't like, and happy if you declare war on them. Diplomats can switch this sympathy.
mmm.... I didn't know, then maybe it could be good to make this a bit more 'visible' (and maybe more powerful). Some kind of inform for the King or the Council about the 'morale situation' of the realm.
