In The Lutheran Hymnal [HymnalID: LH1941], the editors describe the authorship of several hymns as "Author unknown, xxxx" where xxxx is the year the hymn was written. That is, they know the date it was written but not the author, and explicitly say so.
In trying to preserve this data in the INSTANCE record, however, we run into a problem. In the INSTANCE record, it appears that one cannot leave the Name and Authority fields blank (the direction given in the wiki for an unknown author) while at the same time entering a Relationship (in this case, author) and a Date. Thus, leaving the author field blank eliminates the ability to keep the given date of composition for these hymns.
Would it be helpful to create a generic "Author Unknown" person, with a description that says something like "In some hymnals, the editors noted that a hymn's author is unknown to them, and so this artificial "person" entry is used to reflect that fact. Obviously, the hymns attributed to "Author Unknown" were written by many people over a span of many centuries. On the other hand, if a hymnal simply did not list an author for a particular hymn, nothing will be entered in the Author field."
Just a thought.