I made this idea into a small mod, or rather updated an old mod that did something similar.
- Ordnance ML 3-inch Mortar Team:
Buildable in British T1 for 240MP. Slower rate of fire than the German GrW 34, but more accurate. White Phosphorous Barrage at Vet 1. Counter Barrage unlocked with the Advanced Emplacement Regiment.
- Artillery Pit:
Replaces the British Mortar Emplacement. An empty structure that can be built by Royal Engineers for 100MP. All mobile mortars and artillery pieces can be garrisoned and receive a 15% bonus to the range of their barrage abilities. When empty, the structure can be garrisoned by the enemy. (Might be restricted to mortars only. Still playing around!)
- Known Issue:
Artillery guns and heavy mortars can only fire to the front side of the emplacement, because they can't move or rotate while garrisoned. I'm not sure if this can be changed.
It's available on the Workshop:
http://steamcommunity.com/sharedfiles/filedetails/?id=679152285
Thanks against for making it into a mod to prove a point.
And I also think the heavy guns are plagued by the same bug when garrisoned in emplacements, that's why it was removed.
But in theory they can still perform well when emplaced, they'll just be more limited than the mortars when inside.