I think that might be beyond the scope of a bookmarklet - It has almost no long-term memory; I don't think it has any ability to know what you have open in any tabs other than the current one, or to "save" information between uses. (Well, you could maybe have one that puts tickyboxes on your reading list to reblog several entries from the same page of the reading list? But that would be of limited usefulness.)

/I say, pretending I actually know things about bookmarklets

You might have to jump all the way to a browser extension for that, and browser extensions are definitely beyond my scope!