Hi Stefan,
Temporal AA should work with scene capture. In the SceneCapture2D, you’ll need to ensure that the capture source is set to “Final Color (LDR) is RGB” in order for postprocessing to be applied (otherwise the output will be the raw HDR frame without postprocessing). In the postprocess settings for the capture component, you’ll also want to check that TemporalAA is enabled (Misc->AA method).
You should be able to avoid duplicating the stage geometry by using level streaming. You can create a separate streaming level for your stage and stream it from any of your game’s persistent levels as required. There’s some more info on level streaming in the Level Streaming section in the docs.