The way I’ve used it in montage in the past is to create a segment in a clip, “clone and substitute” the segment, and then “edit source”, from which you can send just that selection to RX and back using render in place. Or do the same for an entire clip, using “edit source” or “clone and substitute”/“edit source”. I always use “clone and substitute” first for safety, so I still have the original file as it was, if needed.