Mackey does have a point though. It would be cool if you could reinforce an army like the Elves did at the Hornburg in the movie.
Maybe it could be coded that it is possible for allies to defend custom settlements inside of the walls. It will probably need some toying with the deployment areas. I don't know how to do it though, I think the only 2 people who do know how to do it are Leo and King Kong the man(or ape) himself.
But it will need some serious tweaks. If the AI is the reinforcing defending army, then it will need to understand that the situation is changed. It will probably take a long time to code, although it is something nice for the future.