The Moulton bike can be taken apart for travel (just using it as an example, I don't have one). The cables that run from the handlebar to the rear triangle are not single continuous cables but are made of sections which are connected together by a sort of threaded "union" of some kind. What are these cable-union-devices called?
Cable Splitter is the name you're looking for.
Another brand name is "Easy Split" but that's essentially the same thing.
JTek has some that look like this:
The end on the right hand side is clamped down with one or two tiny grub screws, which feels like a weak point. Plus they look to scratch up the frame's paint at that point, and make a tap-tap noise on every bump.
Another link of relevance: http://stashablesteeds.tumblr.com/post/40348418610/cable-joiner-designs
A competent machinist could make something like this, possibly starting from a chunky Chicago bolt, aka sex bolt and then making some way to secure the wire coming down from the bars. Even a permanently silver-soldering the inner wire to the fitting might work, and if it breaks while out, you'd just fit an entire spare normal inner cable.