How much setup time are you allowing the AI to do this.
If its alot, you can precalc paths (ie- some monte carlo random sampling to feed start+end points to build up a routing network map)
Will the AI be given data containing where the enemy is starting so that is can figure which way THEY have to go to get to where the AIs units will be ??
Then you can project where they can get to so the AI can pick good defensive positions (if that is to be the strategy) or where good ambushes might be probable or even just which path is best for a Zerg Rush of the enemy (if there is a target that must be taken ...)