@mattscomments No, not a plugin (yet?). I exported them via the Amazon Kindle Desktop app and added some CSS to improve the mark up a tad. Doesn't take too long though. I greatly prefer to own this type of content on my own site first and then syndicate it to places like GoodReads after the fact.

I've written some details here: http://boffosocko.com/2016/10/24/notes-highlights-and-marginalia/
Searching my site for "marginalia" will uncover other resources: http://boffosocko.com/?s=marginalia

I also recently helped guide Jeremy Cherfas though some of the process and he's well documented a similar (non-WordPress) workflow here: https://www.jeremycherfas.net/blog/setting-my-marginalia-free

Given your Twitter background at @mattmaldre:
There are some in the indieweb movement working on notes, highlights, marginalia, fragmentions, etc. You can search/find wiki pages for those topics here: https://indieweb.org. In particular, Kartik Prabhu has a fantastic set up which he describes (and you can see implemented along with sample code) at: https://kartikprabhu.com/articles/marginalia

I presume you'll already know about https://www.w3.org/blog/news/archives/6156? As well as work by groups like http://hypothes.is.