At the moment the only two things that distinguish units that have seen a lot of fighting from those who haven't is:
A - a high captain level (only affects leadership)
B - the possibility of getting 96% training instead of 85% (which is the max from training as far as I know).
So, why not have another aspect that affects cs and combat reaction: courage.
I envisage it as being another variable in addition to the training/morale/cohesion/equipment equation. Some of it could be gained from training, but it would have a much lower cap than training (say, 50% or lower). It helps to know that you are skilled, and the mate on either side of you is also skilled and won't run away. However, the true test of courage, and strengthening of it, is in battle. So, to get up to higher courage levels you would need to get them battle hardened. This would help distinguish between those who have just spent their time training with lead-filled wooden swords, and those who actually have seen the whites (or reds/greys, depending) of the enemies eyes, and seen them off.
Thoughts & suggestions?