I tried to export a separate pass for the RS Sky Object, but neither the RS Object Tag with an Object ID, nor a Puzzle Matte, nor a Background Object gave me a satisfying result. The sky isn't seen by the cryptomatte. And an alpha channel is also not produced. Using the override "force alpha to zero" on the Sky Object itself came closest to what I'm looking for – but unfortunately, it also breaks the beauty pass.
Would be nice if so can help me. THX