Make unit invisible for real

[Gunfreak]

Yet again, I have a plan for a mission, yet again it doesn't seem possible to do.

I have a nice factory building on Saipan, perfect place for the Japanese to hold up and fire on the advancing Americans. 

I want that building to fire on the enemy, to simulate some MGs holding up there, however that doesn't seem possible. as any units I put there will just be on the roof and look ridiculous. I can hide it for infantry or the map, but I can't make it truly invisible. 

So that is a core feature I request. Even better, if I can simply make static buildings on them map fire guns. Like Add a zone over the building and have bullets come out of the zone.