I had had a good look around the the sunroof motor fittings and it looks like there is room just in front of it. There also seems to be enough space (depth) just there between the actual roof and the head lining.
Then I think you'd get away with popping off one of the 'A' post finishers inside the car, and feeding the coax from your drilled hole to the 'A' post, then just run it down inside the finisher, a quick shimmy and you'll be behind the dash. Bingo.