In Windows - create a shortcut to the source code folder - rename it so that it has EXACTLY the same name as the book has in the Calibre Library, e.g. "Fred Bloggs - Programming in Binary.lnk"
Configure Add Books to Automerge new formats
Now add the shortcut as you normally would. It will appear as a LNK file in the relevant Book Details panel. If you click on the LNK format in the Book Details panel your source code folder will open in Windows Explorer.
I'm not sure if support for LNK files is documented anywhere. I looked but didn't find it - but...
OS/X Alias files are similar to Windows shortcut files
Linux Softlinks are vaguely similar to Windows shortcut files.
If OS/X Alias and/or Linux Softlinks don't work, then you can put a hyperlink to the folder in the Comments or in a Custom Column of type Long Text. Of course you can also do that in Windows if you prefer.
BR
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