Haven't implemented this but I have considered it. My thought would be to loop through notes and run the script (which will extract embedded PDF text) and then when it's done with the OCR, add an ocr tag to the note to indicate it's been done. I don't have the bandwidth to add something like this right now, but would certainly welcome a pull request! But yes, as a workaround you could extract the PDFs and let rest-uploader bring them in, create a preview image, and extract the existing OCR from the PDF.
For all the Evernote users out there that are jumping ship (including me!), this would be very welcome. Your script is working well for PDFs and images as they are added, but both I and many others have a large archive of notes that pre-date our use of Joplin.
If you or anyone can do this, I have a suggestion: provide a flag to go through all notes and process PDF files, or all notes with a particular tag. That way, a user can tag some notes with images that need OCR, and the script doesn't OCR every image, some of which won't need it.
Any chance this is on your near or medium-term radar?