What I came up with is a position:fixed div in a tiddler at $:/tags/BelowStory. It referrences a $:/state/popup/ tiddler for the pinned title, so changing the pop-up reference does not affect the save state. It is the same width as the Sidebar, so the Sidebar is just being hidden when the pop-up is shown. There is a ViewToolbar button named "popup" that does the pinning.
https://www.soyawanna.dev/Browser_Editor/Tom%20TWPages.htm
If I worked to around to append a list of tiddlers, it could resemble side-by-side story rivers. If I change the size / position, I could have several pinned tiddlers floating above the river like post-it notes.
Does anyone know of a plug-in that has been developed to do something like this already?