Hello David,
WELCOME TO THE AWS WELDING FORUM!!
The best way to answer your question is to make sure you have asked a few of yourself and your project.
I presume from the tenor of your question that D1.1 is the prevailing applicable code, so the most obvious starting place is taken care of.
Then, I would presume (never assume) that your engineered and approved drawings are calling for this particular material on part of your project? Again from the tenor of your question it appears that is so.
I presume there will need to be welding involved in the attachment of this D plate to some other member? Is there any way to bolt it instead? Is it actually part of the structural assembly or is it classified differently so that D1.1 would not apply to it?
On the surface it would appear that your conclusion will prevail and you will indeed need to test to qualify your procedure. With PQR in hand you can write your WPS and weld away on your Dplate to whatever other member you have. The testing should prove to be relatively painless but it will cost a few bucks and take up some time.
There are aspects of this that we don't know, but on the face of it I would venture to say 'Yes', you need to qualify to Clause 4.
He Is In Control, Have a Great Day, Brent