Weapons that fire over walls but not through ceilings?

[erdrik]

I want to make a grenade launcher that can lob 'nades over walls, but only if there is no roof over the shooter.
I have not touched on the C#/DLL aspect of modding, and have thus far stuck to XML editing.
I would prefer to keep it that way, but Id give it a go if it is required.

Is this possible? Any information would be appreciated.