I have tried to tinker something up for doing just this, also a couple of persons have done something similar before.
One where a alot of code was changed and one where just a small fraction was added.
But i didn’t get it to work 
What i yearn for and have tried to fix is a slaved Z axis that works on the Cartesian styled z movement and in tandem or parallel to to deltas Z movement.
Z hops and other smaller variations can be ignored, just the current Z layer and homing in tandem is what i was aiming for.
Even fiddled with manual stepper and tried the gcode aproach, but obviously in gcode approach, the axis will not home in tandem/parallel to delta’s Z, so if minimizing cable length/bowden, this would not end well.
So for now i am giving the idea up and hope someone else would be willing to try it 