Is there any open source library (or even blogging platform) that supports this? Or writeups of how to make the comments 'stick', even if a new paragraph/sentence is inserted?
My naive plan was just to give each comment-able item (eg. a paragraph) a specific ID, and having a many-to-one relationship with comments to paragraphs. I never followed through, so I'm not sure what pitfalls you'd likely encounter doing it that way.
Your naive plan was mine too, but... then I realised that for feedback while drafting, I'd be breaking up paragraphs (with comments on them), adding new ones before commented paragraphs etc. So it never happened.
http://blog.bitbucket.org/2012/10/30/building-inline-comment... seems a far more technical (and reliable) way to handle these situations.