Another option would be to look into embedding a topic on your document page. It would require the user to click into the comments to leave a reply but that’s no different than any other commenting structure.
I have created a prototype to showcase the idea. I am in the process of simplifying the UX, re-engineering the whole stuff and planning for version 1.
I would like to pursue the iframe idea further :
Has anyone embedded a topic list instead of a comment list? What if I use categories as unique identifiers for document sub-parts? I could then use the appropriate url (for example : https://mydiscourse.com/c/document-sub-part-id) to show only the relevant topics. But will it then be easy to hide all the unnecessary banners and buttons to show only a clean topic list ?