I'm doing this with the fork installed, so that was my first mistake.
I tried feeding in a cable from the top tube then hooking it with a piece of wire. I tried the shopvac and string method. I tried taping a piece of cotton to the cable end then hooking that.
Ultimately what worked was a set of neodymium magnets. I carefully threaded the cable down to just above the downtube hole. I could look into the bottom bracket cable port to see where the cable was, and I could also follow the cable location with a small magnet on the outside of the frame. Once the cable was positioned just above the downtube hole, I was having trouble getting it over a lip in the carbon fiber. I stuck some magnets to the end of a 30mm long screw, then used the temporarily magnetized screw to fish out the cable. I had a friend slowly feed the cable into the top tube while I lay on my back under the bike and drew out the screw from the frame.
I found the magnets at a local hardware store for ~$10.
If I ever have to run it again, it would probably be much less work to just remove the fork and run it in reverse order like Dave suggests.
edit: Dave, can you provide details on how you mated the Jagwire Links housing to the ferruled mousetails included with the frame? I can see how traditional 4mm housing would fit into the mousetail's ferrule, but the fit of the links is not as secure there. I could put a link directly onto the lip of the mousetail ferrule, but it looks like it could rub noisily at that interface. Maybe it's not something I need to worry about and I should just use the link, but I'd like to hear what your experience has been.
edit2: I accidentally let the cable fall all the way into the toptube hole, so I had to run the rear brake route again. I removed the fork and did the reverse route, finishing the job in about 10 minutes no problem.