I'm very interested in this. I read a lot of books that aren't from Amazon on my Kindle. I do a lot of highlighting, which I like to export to readwise.
In order to do this I have to plug in my Kindle. Mount it. Find the clippings file. Upload the file to readwise.
The best I came up with was a script that detects when I plug-in my Kindle and opens the finder window and opens the readwise page. Pretty lame.