Originally Posted by kiwidude
As you have discovered Calibre has no direct support for audiobooks. If it consists of a single mp3 file, you can just drag/drop this into Calibre like any other book format, it appears in the list of formats in the book details window etc.
So I can't think of a direct way for a plugin to help you. I would suggest posting in the main Calibre forum and asking others if they store audiobooks in Calibre and if so how they approach it.
I have several, that were purchased accidentally, and expect to add data on them to Calibre, but they are actually stored with my Musicmatch 9.0 which makes more sense since it is intended for handling audio.