On a few that I have worked on, the coil forms are able to slide along the ceramic shaft. Although ceramic is brittle, it is far stiffer than most of the alternatives. The sliding contacts do put a bit of upward force on the shaft, so it needs to be stiff enough to avoid significant upward deflection. Fiber reinforced plastic might work, but I don't think any unreinforced plastic is going to do the job. I agree that they did have alternative materials (like fiber reinforced bakelite) that would have been OK.
I noticed while investigating the various designs of Inductuners used by DuMont that they did a redesign somewhere in the middle of the production run of RA-103s. One design has the shaft sticking right out the front of the set going to the user knob. These are the ones that break, since it is easy to apply pressure to the shaft in shipping. In the redesign, the shaft goes only to a gear, and the user knob shaft is a metal shaft on a mating gear. This version probably never breaks. See here:
http://videokarma.org/showthread.php?t=262692&page=3 .