.... the idea is to make a combination of threaded and non threaded headsets/stem combination.

The idea is to make a new shaft for the forks that has both the threads for the threaded head bearings and a extension that comes out the top for the stem from a thread less bike can clamp onto. This would leave the inside clear. I could take the cables from the inside of the bars into the stem then into the new vertical shaft...... and down.
Going that far sorts out cable 1 and it gets it to the bottom where I can go into the fork leg out if sight and down to the brake.

But cable 2 and 3 need to go out of the stem and head down the lower frame tube to the BB and beyond..... my idea it to make a slot in the tube for the cables to come out the back of the tube and re enforce the tube to take the strain by building it up inside (could be practically solid with passages for the cables at that point)...and outside it as room allows.... then as it is just a road bike, restrict the handlebar movement to stop the "slot" going past a certain point where the cables would tighten or get damaged.

Thoughts? What am I missing??