Thank you so much @reosarevok for the info! However, the artist does not want to publish the scores before any release of his works, but he does want to list his works here at MusicBrainz.
In my country (Netherlands) we have BUMA/STEMRA and there all artists can register their original compositions and every composition gets an official unique work number just like ASCAP does for United States composers. So what I was thinking of, is adding the works to the artist and then at the work-edit page at the tab work attributes link to their unique 9-digits work numbers to proof the existence of the works. But BUMA/STEMRA is not in the list of identifiers yet, to do so.
@Freso sent in a ticket on October 23, asking if BUMA/STEMRA could be added to the list of identifiers, so that artists from the Netherlands can link their works to the official BUMA/STEMRA unique 9-digits work numbers to proof that their works exist.
So I was wondering, would you have any idea of how long it will take, before BUMA/STEMRA is added to the list?