DVDFab is one of the few programs I know that intentionally misuses the term rip. It generally means moving the data from an optical disc to a hard drive.
In your use of the term, I take it to mean that you want to change formats from DVD-video to MKV without re-encoding the video and audio, since you mentioned passthrough.
If no one here in the DVDFab forum can help, send me a PM and I can direct you to another option (free), that I can’t mention in the DVDFab subforum.
But perhaps someone will be able to help. It seems unlikely that public domain movies would be encrypted, so the usual problem of poor decryption may not apply here.