I'm not sure I quite understand the question but it sounds like you might be having two problems: 1) figuring out how to string the cables through the enclosed fuse (normally cables are installed before covering) and 2) dealing with the fact that the cables can't run in a straight line.

As far as the first problem, I think I'd suggest putting a small fishing weight on a piece of thread and lowing that through the exit holes down to the servo location. Then attach the cable to the thread and pull it back out. Unfortunately, this is only half the problem.

To avoid allowing he cable to rub against bits fo the fuse internior full sized aircraft used pulleys. On my EIII I just used small eyelets. Unfortunately, I don't see any way of installing these without taking off the covering.