If you're on a Windows system then you can create a shortcut to the supplementary folder for a book within that book's calibre db folder. Select a book in your library and use "Open containing folder". Create a shortcut to the supplementary folder.
This shortcut can then be accessed again by using "Open containing folder".
One point to watch out for is that such shortcuts are not kept when copying the book to a different library. There may well be other caveats.
(This is of course 'bad practise' in the sense that it involves messing about within calibre's db folder, but it works for me.)
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