Why not copy the epub into the book's data folder? That way you have a backup and don't need to use original_epub.
The quickest way I can think of to copy a format into the data folder is:
- Use 'O' to open the book folder.
- Open Edit Metadata / Manage data files.
- Drag the epub from the file explorer window to the Manage data files dialog, which copies the format.
FWIW: I'm not interested in adding the option. @kovid might be but you would probably need to make an enhancement request on
https://bugs.launchpad.net/calibre.