Does "pendulum effect" apply to hang gliders or any aircraft?
For a proper discussion, we should first define what a pendulum is. Only then can be established if such an effect can exist in airplanes.

Let's base the definition on Wikipedia. It says that


  A pendulum is a weight suspended from a pivot so that it can swing freely.


Maybe it is also worth to look closer what a pivot is: A thing on which something turns.

So the pendulum is fixed to a fulcrum which keeps it suspended and allows it to swing freely. The ideal pendulum has all its mass in its massive bob and, therefore, pivot and center of gravity are not in the same place. If the center of gravity and the pivot would fall together, a pendulum could only rotate but not swing. And that swinging motion is what the pendulum is all about.

Now for airplanes: Here we have no pivot. All rotation can only happen around the center of gravity. This is equivalent to the pendulum with no length which is no pendulum any more. But what about hang gliders? Using Pilothead's sketch let's consider how a hangglider starts a rolling motion and compare that to how a glider does it. The top two sketches show each in steady straight flight while the bottom two sketches show both initiating a roll to the right:



In both cases, a lateral imbalance is needed to create a rolling moment (red round arrow). While the pilot of the hang glider shifts the whole wing sideways, the glider pilot commands a lift difference between both wings using ailerons. Note the shift of the hangglider to the right and with it the center of lift in the lower left sketch: The center of gravity stays where it is due to the conservation of momentum while the pilot will shift slightly to the left.  In case of the glider, no such lateral shift of the craft occurs; instead the distribution of lift is shifted in order to create the imbalance. The effect is the same: A lateral displacement between weight and lift which causes a rolling moment. Rolling happens around the center of gravity (because of conservation of momentum again) and in all cases there is no displacement between the pivot point and the working point of the weight, because both are the same: The center of gravity.

A hangglider is only superficially different from an airplane because the center of lift is actively shifted by displacing the wing laterally instead of a change in lift distribution (neglecting the influence of the pilot mass on the rigging of each wing), but in all cases the vehicle will rotate around its center of gravity. Weight has no lever arm to that center of gravity, so there can be no pendulum effect. Or keel effect, fo that matter.