The question is if designing a tapped hole on the flat head which is plugged with the proper pressure plug, is against ASME Sec. VIII Div. I? is there anywhere in the code that refers to such a design possibility?
Sect VIII Div-1 does not explicitely approve a number of common features. So, don't take a lack of an explict approval as meaning that is is not allowed.
It will just be a flat head with a nozzle [hole] drilled in, and no pipe welded into the hole. Makes the calc pretty simple, as there will be no reinforcement area available other than extra thickness of the head plate.