How about an option for 'By fiat'
What I mean is that IMC, whever there is mass battle, dice don't determine the outcome. I do. Whatever needs to happen, happens.
Now where do the PCs fit into this? Simple. You just determine that if PCs accomplish mission A then result B happens. If not, result C happens. An example
The heroes have been commanded by the military forces (assuming they are part of the military) to deliver a message to the field general. They have learned new information. It seems that the enemy will be using tactic X, and unless he retaliates with tactic Y, the Eastern army will be crushed. FOr instance, maybe they will fire crossbows into the pikemen before the cavalry charges. Unless they have heavy shields ready, the front lines will be broken. Or whatever flavor text you prefer.
Now they must go on an adventure through wild and/or enemy territory, evade capture, outwit spies, etc. to meet the general before the battle. Adventures on a timeline are always so exciting!
Nowhere in here do I have any need of dice to figure out what happens in the background. If the characters succeed in their mission, the Eastern Army wins. If they fail, the army is decimated.