This seems to be more of a bug with armies joining the reinforcements for city defense, rather than subverting. One of my generals had the "night fighter" trait so I thought i'd pull a classic TW move. Attack a city that is relatively lightly defended, but has a big "reinforcement" stack close by, swap to night attack, and negate the latter.

Even though the enemy tab was greyed out, indicating it wouldn't join the fight, they still did. And i had to contend with them in the siege itself. As of now, if the enemy has an army near a city, I don't think there's anything you can do to stop it from reinforcing the seige.