While the $:/tags/AboveStory method fits your stated goals, you might also consider Hidden Setting: Content to be displayed for empty story, which allows you to designate a “falback” tiddler that is only rendered if there are no other tiddlers shown in the StoryRiver. As soon as you display other tiddlers, the “empty story” tiddler is no longer shown.
Another possibility is: TiddlyTools/Pin, which provides a tiddler toolbar button to interactively select one (or more) tiddlers to be temporarily rendered “AboveStory”. When “pinned”, the tiddler can’t be closed until you click the toolbar button again to “unpin” it.
-e