Another "why didn't I think of that!!!" idea from Henke on the Classic & Vintage board:

"I have a bike with internal routing. When the time comes to change cables, I just remove the seat with post and use my index finger to point the new cable up the hole from inside the toptube. I have never had a problem with this method. Guess you need quite long and slim fingers, though."
http://www.bikeforums.net/showthread.php?t=360517