Assuming the scipts are more or less the same as on the Public Release, Siege Weapons to not care about pesky little things like Line of Sight.
It can however easily be implemented.
To you scripters - try using ActionCastSpellAtLocation() instead of ActionCastSpellAtObject(), with the Location being the location of the targeted object.
You might get better results this way, atleast the VFX should fire more often.
However, this will make it possible to avoid being hit by the AoE from the Siege Weapon, if the target is moving (fast enough) when its location is being fetched.
It would also make some Siege Weapon attacks easier to avoid, since the different projectile types (accelerating, homing, linked, ballistic, spiral or bounce) all have different speeds/delays.
It might be neat but it might also not be preferable, depending on the designteams preferences - hence why I decided to go with ActionCastSpellAtObject() in the first place.