I don't have time to model any of this, but by replacing the inner rod with another tube and mounting the arms, tubes and motos on a "riser" that has a threaded hole in the center, the Z-Axis motor could be mounted above the assembly with the threaded rod going down the center of the tubes. Using a larger threaded sleeve on the riser would add stability as it moves, and in an ideal configuration a Teflon sleeve could extend all the way up the inner tube providing the greatest support. Of course, that might just be overkill.
I feel like it's almost Mad Science time.
I feel like it's almost Mad Science time.