How do you insulate the top of a walled dead space that was made for an insert fireplace's pipe run?

Go to the attic and cut a piece of cardboard to fit around the open portion of the chase and seal it off there. If you attempt to fill the space you may create a fire hazard. It will not take much to block that but be careful doing things to fireplaces especially if the home is not yours. If there were ever a question with a problem and it links to something you did you could get introuble.