The new Plex "Categories" tab includes a list of all genres in each library. This seems like it could be useful for adding non-standard genres as well, to distinguish some media categories from other types of collections. For example, one could add "genres" for:

Ideally, this would be a simple command in any builder, similar to "item_label:" or "item_label.remove:". In this case, it would be "item_genre:" or "item_genre.remove:".