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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.
Roll stability, as in an opposing aerodynamic moment caused by a roll attitude of an aeroplane: no there is not. These stabilising roll torques are a result of indirect state variables:
Yes but the effect is very weak or negligible on a high wing aircraft. Like a lot of things, the effect is easiest to visualize by taking it to the extreme. Like with paramotors, which get just about all their roll stability, as well as their ability to bank into turns by skidding, from pendulum effect.
Wing flex contributes to better spiral stability as far as my simulations shows. But I guess this comes down to dihedral and sideslip again as well as the "pendulum effect". Just imagine how much harder it becomes to roll an aircraft that flexes the wings like that
Imagine "outriggers" that are inverted airfoils, mounted far outboard of each wingtip. In general, if an aircraft is banked, it will turn, which will mean that the outboard wingtip is moving faster than the inboard wingtip. In this case the "outrigger" on the outside of the turn will generate more downward lift than the "outrigger" on the inside of the turn, creating a roll torque toward wings-level.
This dynamic seems to play a key roll in explaining why hang gliders and "trikes" tend to experience increased roll stability (or decreased roll instability) in flight at low angles-of-attack (high airspeed), despite the fact that at low angles-of-attack, the dihedral-like "downwind" roll torque contributed by the swept or delta shape of the wing in the presence of sideslip, is much less than at higher angles-of-attack. By "increased roll stability (or decreased roll instability)" I'm referring to an increased tendency to roll toward wings-level or a decreased tendency to roll toward a steeper bank angle. This is static stability, not dynamic stability.
Note that these aircraft have lots of "washout" and the wingtips are generating downward lift during flight at low angle-of-attack (high airspeed). In some cases the lower "flying wires" can observed to be slack.
In some cases the "outrigger" effect described above appears to contribute to a dynamic yaw-roll oscillation in these aircraft. This oscillation may be fundamentally different from the well-known "Dutch Roll" oscillation that swept-wing aircraft are sometimes subject to, typically during flight at high angles-of-attack. For example the timing of the point of maximum sideslip, in relation to the point of maximum bank angle, may be very different in the hang glider/ trike case and the classic "Dutch Roll" case.
For related content on stability and control in hang gliders and "trikes", but not dealing with the washout / "outrigger" effect, see this answer to the related question Does "pendulum effect" apply to hang gliders or any aircraft?
