Panasonic/Tange brake cable guide removal and replacement
This is going to be hard work given you want a bare frame.

If it were a simple swap of cables, you'd feed the new outer onto the old inner, and pull that up.  The new outer would follow the end of the old outer, and it would find the top exit hole because its on the old inner wire.

Once the new outer is in place, you whip out the old inner, and then feed the new inner through the new outer and connect like normal.

You basically don't know if the outer that passes through the frame is encased in a guide tube of some description, or if its just bare in there.

The only way to try this would be to hold the outer at one exit point and try and shove more outer in at the other exit point. 


If there's a constraining tube it will be impossible to push any more outer into the tube.
If there's NO guide tube in there, you should be able to push several centimetres of additional outer into the tube where it collects in a mild corkscrew shape.


Finally - it might be a good excuse to go buy one of those endoscope cameras... I've wanted one of them for a long time but never had a justification.   You could whip out the headset and look along the tube to see what you see.