I think you can remove the rear view mirror assembly and put that piece into position. My memory is fuzzy, but I think I forgot this on my car, and put it in place after the fact. What I can't remember is if I was able to get to the screw, or if I installed it with a little bit of adhesive. I'm pretty sure the screw is not accessible after the fact.
I seem to remember putting the rearview mirror boot/visor clip in place, seeing a gap between the boot and the header molding, then having an "oh, sh*t" moment as I remembered that rubber block.