Just generating a unit every x turns would be doable, basically just add a new random event that is called from the doTurn stuff either in the .dll or in a python file - this can easily check for AP or UN in the city and can be timed to occur every x turns. The thing about control only with approval would be tricky, I would probably try to add a new resolution which allows this event to occur. The problem with stuff like this would be to teach the AI to understand this resolution.
In any case you'd need to do at least some python programming, though there are templates in the code so with some tries it might be easier than it looks, the consent of all other members would be tricky...
Short answer: no there is nothing programming wise to stop this, its just some effort involved and it is not just done by changing some xml files.