The code does not allow you to install a horizontal screen over the opening. I'm wouldn't be concerned of an opening in my home because I'm not tolerant of vermin in my attic. Of course, I'll correct something if its a real issue no matter how long it's been there, but this furnace closet has been like this since the 1950's.
The head inspector re-inspected the job last week. He was the one I was dealing with but neither of us had actually seen the job. Apparently he passed it and signed off on the permit.