Hi, Bugbear – don’t give up after learning so much about the subject!
You don’t have to make an application to the Lord Lyon (or, for that matter, to the Chief Herald of Ireland).
I am sure one of the US registries will be only too glad to help you.
Not sure what you mean by substantive arms – the chances are good that you will be able to devise a variation on your clan arms that are sufficiently different for you to avoid the accusation that you are “stealing” someone else’s arms.
But at any rate I am glad you have a better grasp of the subject.
Regards,
ike