If I as an admin create a content group and try to link to a page in an archived space, it will not be found. Since spaces and pages that contain the source page excerpt macro for this use case should not be found in the normal search, the interface should support searching in archived spaces.
One possible solution would be to allow for the alternate notation <SPACEKEY>:<pagename> (like in the "include page excerpt"). As of now, this will lead to a failure when saving without an error being displayed.