@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:/
Searching my site for "marginalia" will uncover other resources: http:/
I also recently helped guide Jeremy Cherfas though some of the process and he's well documented a similar (non-WordPress) workflow here: https:/
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:/
I presume you'll already know about https:/