I talked to a guy at the NOVA Star Party this past weekend who had modified his 8" reflector to handle that problem:

First he modified the rings so they were loose and allowed the tube to be rotated easily without loosening anything. So it would slide easily, I believe he used teflon grips. Then he basically attached a retaining ring so that the tube wouldn`t fall out, and used teflon to allow it to slide against the old tube rings.

Playing with it in the light, it seemed to hold position well, but rotated easily and smoothly when you wanted to move it.

I suggest you email him for further advice on how to rig that up: mjmills@fpcc.net (Mike Mills)