The below won't solve your problem, but you might be able to put the info to good use.
Originally Posted by Philosopher
In fact I have been torn in some cases - such as with course guuides for video courses or audio courses - of putting the guide in calibre - because of losing it in the directory where the relevant files are. Sure I could know to search calibre for it - but would have to create some kind of file to tell me there was one. Or I could put a link to the file in the calibre library - but that is always changing (at least for some time). Or I could keep two copies - but taht seems wasteful.
Couldn't there be a way to browse and input a directory into a field.
If you look at the book details window you will see Path: Click to open
Hovering over it will tell you the path to the directory in a tooltip. Clicking this link will open the directory where that book is stored (see attached).
There is no reason you can't place the associated files for a ebook in that directory. Also using the "Open with" GUI plugin
you can set up a right click that will allow you to open that book in an external program (see attached for how my plugin is setup).
I know this seems backwards for Video courses but some books come with small linked multimedia files and the linking works easier with the associated file(s) in the same directory.
Just some thoughts. Since I haven't had the need to try this it might be rubbish.