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.
It’s not exactly answering the stated criteria, but it feels close enough to mention as a relevant option - the “pincard” solution as described in the "prerelease animation’ thread. It’s based off the “Hire Jeremy” animation at the top of tiddlywiki.com
pincard:
appears by default, with unique styling
can be closed, but then a mini version appears in the sidebar
defaults to on top - it can be shuffled in order by drag-and-drop the open tids, but will reappear on top if closed and re-opened from the mini view in the sidebar