We've been discussing a note-only export plugin. Currently the idea is to let you chose between plain text, HTML, and markdown (plus item/photo metadata in the front matter). This way you'd be able to export all notes of all selected items. Such a plugin should be fairly quick to write, but other suggestions are certainly welcome.
If you need the notes in plain text, but find extracting from the JSON to complicated, it you can also extract the notes directly from the project database using
sqlite3 project.tpy "select text from notes".