Suggested variant rule.
“All mortar units have one Smoke round at the start of the game. Place a small coin beneath the unit to represent this.
The Smoke round may be fired instead of an attack by the mortar during any assault phase. It need not be fired at an enemy unit, but can be placed in any hex the mortar team has a line-of-sight to within the mortars’ maximum range.
Place the coin in the target hex. It will cause no damage to any unit, but all units in that hex automatically count as in cover against attacks from outside the hex - even if the hex is open ground - and line-of-sight cannot be traced through or out of that hex.
The effect lasts until the end of the next assault phase, when the coin is removed.”