I guess it would be a DM's call. I suppose that the restriction put on the creation of such weapons is not only flavour, but also to force the crafter to come up with good protection mechanisms against the dangers of the shadow plane (i.e. make it harder to craft).
So you could rule that some areas within Chaulssin are "manifest zones" within which the properties of the plane of shadow applies. In that case these areas have all the traits of that plane and one would need to organise appropriate protection against negative effects. But you could then craft these weapons there.