This would not be trivial to code, but it would not be fantastically difficult, either.
The combat code already has to keep track of how many people are fighting in a given line, and prevent more from joining in the fight if there's no more room; adding in the extra possibility of there being simply too many people in that line to move forward at all wouldn't be hugely complicated, I don't think.
However, you do have to watch out for potential exploits in this.
If, for instance, only a hundred men were allowed in a single line for a battle in the mountains—what happens if one side has a single unit that has a hundred men in it? Melee combat happens between two units in the same line of battle; how does the game allow any melee combat with that unit?
Just various stuff that would have to be thought about if something like this were to be implemented.
Conceptually, though, I do like it, and have mused on stuff like this in the past myself.
