This is really needed.
As I mentioned to @quorm on the Curio forums, my solution at present is to export from MarginNote as dox, convert that to Markdown using https://word2md.com/, then tidy that up (in ways depending on where I want to use the material) using BBEdit. It’s a bit long winded, though the BBEdit stage can be largely automated. It’s just occurred to me that I could use Pandoc for the docx to markdown conversion, which would enable then me to script the conversion and text processing.