Maybe this digging stick could be given to the [Shorn One] unit which would be a new future unit for the Aztecs as I have suggested before. They could use it when attacking walls specifically, thus giving them an unique attack animation when engaging walls.
My reason for giving them the digging stick is because aside for being shock troopers, the Shorn Ones did also took on special tasks as well as battlefield assistance roles when needed according to what is stated on the Wiki.
If the developers took some inspiration from the “Battle of the Middle Earth” games, then the walls in [Age of Empires 3] could be changed so that they will also function as platforms for infantry units to walk on and where wall scaling type units will be used to ascend to these platforms, just like how it works in “Battle of the Middle Earth” when using siege ladders or siege towers against rampart walls.
A section of the wall could be converted into a type of wall that has a set of stairs behind it by the player that constructs these fortifications.
Though this may be complicated to implement for the game.