Yeah it would have to be a whole new piece of functionality. And to cater for all the scenarios above it would have to be an options dropdown, something like:
If no ISBN is found then:
- Do nothing (default),
- Set ISBN to 'NotFound' if no existing ISBN
- Set ISBN to 'NotFound' always
I leave it to others to comment whether anyone else would find it useful enough to justify the effort.
There is another option. There is already a dropdown option for this plugin which lets it display a search for books that have new or updated ISBNs after an extract completes. I could add another option to this which would allow you to display books for which no ISBNs were extracted. If you want that result to be more "permanent" (as it will get reset when you run another extract) you could select those books and assign some value to a custom column or tag using the bulk metadata screen.
That latter option would certainly be trivial to add if it would be useful.