Is there any effect that contributes to roll stability even in the absence of sideslip? Is there any physical dynamic-- involving high wing placement, "pendulum" effect of CG far below wing, or anything else-- that contributes to roll stability in a way that is NOT dependent upon the aerodynamic forces generated by sideslip? 

For the purposes of this question, "roll stability" is defined as a tendency to roll towards wings-level, or a reduced tendency to roll toward a steeper bank angle.  Perhaps a better term might be "spiral stability".
Buoyancy is one such effect!  Or more specifically, a buoyant force acting above the aircraft CG.  Think hot-air balloon, dirigible, blimp.
I think I've also thought of one such effect involving conventional winged airplanes but will mull it over some more before answering.