Man I'd love this, but it would be adding a whole system that has no precedent anywhere else in the game as far as I'm aware (not that I'm skilled enough to add it even if there was). For how long AoM has been out and how popular modding it has always been, for dream-mods like this I just kind of throw up my hands and say "someone smarter than me would have made it by now if it could be done."

The closest thing I can do - again, not that I'm some great programmer or something, so your mileage may vary - is make units not automatically engage certain other units, like how villagers won't attack other villagers unless ordered, or in self-defense, but that leads to a lot of weirdness when used on a wide scale across military units. (specifically I tested having myth units ignore heroes unless commanded/attacked, and that alone bothered me enough to change it back). But, yeah, that's far from a satisfying compromise.