Kontxt (https://kontxt.io) lets you write inline highlights, comments, polls, etc, on digital content with permission based sharing. You can share full articles with your responses on it (example: https://www.kontxt.io/proxy/https://www.independent.co.uk/li...) or a summary view (example: https://www.kontxt.io/document/d/CChEtxTRf9lt3IZhuTSw1zES8Dg...). Soon we'll add a WYSIWYG editor that you can write your own content and references content from other annotated sources.

Is this the type of functionality you're looking for if it could be self-hosted? How much would you be willing to pay for such a tool?