If I understand correctly farbenfeld is designed for processing images, not storing them. And if you bzip a farbenfeld image you might as well do it with a separate json file for metadata.
No and no.
If you discard metadata (e.g. EXIF from a camera) upon conversion, they don't come back after "processing". And bzip compresses a single file, not two; it is commonly used along with tar to make compressed multi-file archives.
Why would you discard metadata? What I'm saying is you should be able to extract it to a separate file when converting to ff. You're right about bzip and tar, but that just adds one extra step.
