I think this should be part of espionage- not spies, but groups of civilians who infiltrate a country and try to raise unhappiness. They could also neutralise some of a cities improvements.
I thought a form of this was implemented in CtP (the Preacher unit?) but there have been complaints about play imbalance. I'm no CtP player or fan, so there'll no doubt be rebuttals.
Partisans. Infilitrate lines and are resistance fighters.
I bought and played CTP I. I did not like the TeleEvangelist, Preacher, Slaver, etc. Very easily to run through opposing civ and wreak havoc. If I remember correctly, only a TeleEvangelist could see a TeleEvangelist. That never made sense to me. CTP did have a slaver unit, which I did use in a manner you describe.