Actually yes but I think that implementing supply lines mechanic is much more complicated than just creating a zone or object that blocks the pass.Perhaps in the castles that are close to mountain passes we could have a project to build a gate in that pass. Although, in real life, the green problem for an army that crosses that pass, would be that the soldiers of the castle would attack their supply lines or the rear of the army, they could even surround them if they had another army waiting at the end of the mountain pass.