Interesting question. I've not dropped speaker cable down inside an exterior wall before, although my "to do" list includes dropping a network cable down the exterior wall of the breakfast room. When we've dropped network cable down walls with insulation at work (couple conference rooms and the two private offices), we've had to use a fish tape to pull through the insulation, and I'm probably going to resort to that at the house (first attempt with just a plumb bob failed miserably). It's that two foot clearance you mention that worries me - can you maneuver at all in the attic space?

Of course, if you are planning to mount speakers on the outside wall and can get around in the attic enough to pull it off, there's always what I resorted to in our den: mounting the binding post plates in the ceiling. It turned out very nicely, with only one of the four surrounds that required some speaker cable running along the edge of the crown molding.