I think a number of basic ideas used in local mortar support can be applied to artillery support - check out those posts here, here, and here - things like automatic knockdowns/pinned results, chances for suppression and kills, and the effects of heavy cover in some cases. I think fire support from battalion and divisional headquarters should be potentially devastating - and not always welcome.
I think heavy indirect fire could be handled in a manner similar to Flames of War - simply determine a center point for the barrage and measure out a large circle, square, or rectangle on the tabletop and assume multiple rounds pound the area liberally; I think 2' square feet would do nicely. I think any infantry would be automatically pinned with chances to suppress or kill - ie. maybe roll a d10, with results of 4-8 resulting in suppression and 9-10 in heavy wounds/killed. A building might provide some protection, changing the odds of suppression to 7-9 and kills on a 10. In woods, it might be similar, although I recall that timed fuses were sometimes used to cause blasts in the treetops to make things much worse on the ground.
If by some chance either of the platoons on the ground have a chance to call in fire, it will probably be worth having chances of delays, even if requested for a specific turn in the future. I'd say if a defender has the option, it's a good idea to go ahead and identify a limited number of landmarks on which the barrage will center. Offhand, I'd say that planned defensive fire would have less chance of scattering, or at least not scattering as far as artillery that has not had a chance to make some preparatory test rounds.
If the big pattern blast does not appeal to you, then it might be worth trying a series of smaller templates (say, 12" in diameter) and giving them a chance to deviate or pre-assign a pattern - ie. "the first salvo is aimed a landmark A, and the next will occur due North, 12" away.
Of course, the random event table comes in handy here. It may be that neither side has a chance of calling in the heavy guns, but knows there may be a chance it will fall on their behalf. It may be that it does not matter which sides is firing the big guns - the referee or scenario designer can simply pre-plot fire based on certain landmarks and roll randomly which one receives fire when it happens. I'm sure battalion commanders have ordered or requested fire for their men without consulting them. In a lot of ways, I think this may be the best approach to using battalion and divisional artillery support - the players have no real control over it and at best they are only aware it could happen.