I had a similar idea earlier this year:
and this can be achieved today by registering plugins with the commenting system. Read the comments, it’s not straight forward to build a universal model.
The GSoC project app is headless and is focused on extracting the information from headers, but the way we send the information back to clients is sort of universal. Plugins for comments could be written to enhance the files app by letting users add new fields, but don’t forget that NC 10 has a workflow engine with automatic tagging which could be seen as some sort of metadata already.
I think what would help are some use cases and some functional requirements if you can.