This is not printable and just a concept.
I do not have the time to construct this at the moment, so i just made a sketch in Paint ;)
The idea is to keep the heavy stepper away from the extruder, like it is done with the bowden type of extruders to have lower inertia and therfore be able to drive the axis with higher speed.
The problem with the bowden ones seems to be, that due to friction, buckling of the filament and elongation of the tube the extruder response is rather slow and there is a hysteresis.
The solution with the flexible shaft should not have this problem.
I think there will be a bit torsion of the cable. to keep the influnence of this low there should be a worm-wheel kind of gear with high reduction at the extruder.
Flexible shafts exist for the Dremel kind of tools, but i think they are maybe a bit disproportional. Better would be the ones that are used for speedos in mopeds maybe.