Any army or villager goes to the cliffs should have a remarkable amount of damage at least if not die.With this jumping from cliffs means having risk even if u are going to rush.Because u can't climb back.Only siege tower will help you to climb.Or sometimes it will be the short way to catch something.Or it will be the last way of escaping from enemy.
I think this mod will add joy to the game.People will laugh when they incidently fall from cliff.

As a scenario with huge triggering you can do this, but as a mod not possible.