If I am correct, Picard (at this moment) does not read all existing tags that are present in a loaded file and make them available for scripting.
Would it be an idea/possibilty to have a plugin that reads all tags present in a file, and make them available as a hidden tag?
E.g. if there would be a custom tag in a file named ‘subgenre’, it would become available for scripting as ‘_subgenre_file’. (or something like that)