HQs can act as spotters for mortars, as long as the mortars are in command range of the HQ and the HQ has LOS to the target area. Any HQ will do.
You can use pre-planned arty missions by targetting areas during the 1st minute of a battle. You can use the pause command to delay this preplanned barrage. Once set, you cannot change a preplanned barrage later in the game.
You can place target reference points (if available in the scenario or bought in a QB) which act as pre-registered target points. Offboard artillery aimed at those TRPs gets a (much) reduced delay, onboard heavy weapons get a to hit bonus.
EDIT: Jason was, as usual, quicker AND more elaborate.