Many months later I’ve rolled my own solution to this problem with a material function, repurposing nodes from the built-in “InteriorCubemap” function. The important part is the left half, which un-distorts the reflection vector and allows one to position 2D textures into a reflection in a rectilinear fashion. The repetitive stuff on the right half simply builds the box with six texture inputs representing the six sides. The “If” nodes crop out the unwanted stuff from each side before the outputs are added up.
Plugging this function into an emissive channel of a material displays the reflection. And then one uses Blueprint to drive an actor’s CaptureComponent2D to generate the per-side 90° views into six RenderTarget2D textures that the material function uses. The result isn’t free of seam artifacts but that’s probably a solvable problem.
