I had thought about a custom trait. Indeed, I have a number of custom items that have a trait associated with them to prevent other races from being able to use them. But having to add a trait to all weapons and armor in the game is only slightly less onerous than creating a custom tech tree.
OTOH: Heaven, I was impressed with what you were able to do in Stormworld with just traits and no custom tech trees.
The other side of this is, I don't want my faction able to produce those items either as I don't want to be able to create trained units with those items nor do I want them showing up in my shops. I suppose custom traits will prevent that as well.