Not possible to effect keyframe animations with HDRI Backdrop in level sequencer; not scene component, not geometry (inherited), not skylight (inherited), not via empty actor (pivot).
Further research suggests a previous method using BP_Light studio but is not compatible with specified cubemaps. As stated in the cross-thread, I am astounded that out of the entire scope of functionality available in UE4 (4.26), it is not currently possible to keyframe Source Cubemap Angle, despite the essential function of the cubemap rotation for lighting a scene.
The only solution I have discovered is to literally rotate the entire game world while the skylight remains in place.
+1 the ability to rotate a hdri backdrop around its z axis is only available manually in viewport. Even though rotation on z in viewport works the sequencer transform equivalent for this actor ‘yaw’ has no affect.
such basic functionality would be good to have
I was having this same problem when trying to light multiple angles for an Archvis project. Took far too long to figure out that the rotation need to happen on the material level. Luckily sequencer has material animation abilities. When tracking your HDRIBackdrop to your sequence make sure to track the geometry component and then track the material parameters. This should allow you to set keys for the HDRI_Angle and thereby adjust your hdri within sequencer. Hope this helps someone.
This works for reflection but does not effect shadows or specular contribution correctly.