Nieman Lab calls out the feature.
And this got a geek going, Daniel Bachhuber, whose new (today) WordPress plug-in implements what he calls WinerLinks, which of course is my last name. Hey nice to get credit for the feature.
The link to a paragraph shouldn't break even if the author adds or deletes a paragraph above. Not sure how you'd do this in a browser-based editor such as WordPress. My implementation is easier because I use an outliner to edit blog posts, and each paragraph has invisible attributes that move with the paragraph no matter how the document is ordered or organized. In other words, paragraph numbers are an edit-time variable, not a render-time variable.
Another feature people should look at implementing in WordPress is sub-text. It allows the author to add parenthetical ideas that are revealed when the reader clicks a little plus sign in the left margin. You can nest these as many levels as you like.
Without an outliner to edit the text, and without the ability to store OPML source text in the WordPress database, I have no idea how you'd implement this, but I'm not a plug-in developer, and have no thoughts of becoming one.