Spotting and information sharing would work in a similar way and units in combat with an enemy unit that they were not aware of before contact would suffer a penalty that troops in combat with an enemy unit that they knew were there would not, thereby simulating a 'readiness for combat' that surprised troops would not have.
Essentially the idea would be to simulate the cumbersomeness of managing an ancient/medieval army but still allow the player to make ongoing meaningful decisions throughout the game.
I like this. I am just not sure how much C&C ancient and medieval generals hand. As you said a game has to be fun though. You could also lose control of your units through retreat or impetuous advance to the enemy baggage train or in pursuit. Underlay it with a CKII relationship and traits system of key commanders of your army/allies. A greedy allied commander would go for the enemy baggage, whereas an ambitious allied commander might disobey your orders to seek glory for themselves.
As a related aside, I'm still watching through the video tutorial series about CKII. One thing that strikes me is that there is much too much emphasis on murder, and it seems it's much too easy for the player to manipulate world events by having tens of characters assassinated, even the king of France, even though the player has the role of some backwater Irish duke. Obviously murder was part of Medieval politics, but if it had been that easy to murder with impunity, society would have collapsed.
Again, restricting the player is key.
I rarely murder anyone in the game. They also often escape murder attempts. You need a high intrigue leader and spymaster to pull it off. I usually try to imprison them and then... take their money maybe make it real hard on them for a while...