With the caveat that I can't see under the engine, as it were, I like the idea. Preserves your arty and enemy intel about it, as well as allowing more options to a side that may have to worry about ammunition conservation. Plus, if the engine can handle it, it would allow for attacking forces to do probing attacks to try and get the defender to commit arty assets in a less important sector, exposing it to counterbattery, etc.
It would have to have a slider of some sort to indicate the level of weapons free that the defender wanted, just to avoid wasting arty on probes that the ground troops could handle fairly easily.
Just my $0.02.