To get the halyard and topping lift lines into the mast, I got a 20 ft section of 1 inch PVC pipe and ripped it three times with the table saw. This set me back three bucks and produced three twenty foot plastic sections that approximated the characteristics of a tape measure. I taped them end for end to create a "snake". This allowed me to tape a length of nylon string to the tip of it and push it all the way into the mast from base to tip. I then used a bent welding rod to fish the ends of the line through the sheeve boxes. When I get my lines, I'll use the messenger string to pull all the internal halyards in place.
I'm not sure how they do it at the spar builders shop, but this worked like a champ.
messenger strings
PVC pipe "tape"
stern view
Tang on the port side boarding steps.