Yes, the regular unfaced rockwool Batts are fine to use above your blockoff plate. I stuffed them up in the open space above my fireplace in the smokeshelf area and around my insulated liner.
I then got some of the foil faced sheets of semi rigid roxul from Grainger and made a blockoff plate with that. I also covered the back and sides of the fireplace with it and fully taped all the seams with foil tape.
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Then, I made a metal blockoff plate and a series of metal panels with 22 gauge steel to cover the back and sides, and painted them with black stove paint to give it a finished look.
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That part is not necessary, especially if you have an insert and will be installing the face plate on it. I had planned on leaving the faceplate off my insert to gain more heat in the room so I wanted the fireplace behind the insert to look decent. 4 years of metal working in a previous job years ago finally paid off.