I have over the past few days tried to use the engineer mortar elite and turrets during defense events in the Silverwastes, and found it a futile effort.
Either the turrets are trashed by mobs in seconds, or they end up hitting outcroppings and edges rather than mobs below the walls.
The husk slingers, a perfect target for the mortar elite skill, spawn outside of the mortars range, even though they can fling rocks with large AOEs inside the forts.