The main issue with this effort, on Wikimedia and elsewhere, will be that there is no guarantee we have any metadata about file attribution and copyright status. See also https://meta.wikimedia.org/wiki/File_metadata_cleanup_drive Even if you made a <link rel="license">-like thing and collapsed all the PD tags under PDM's URL, it would be quite a mess. And Flickr is not so consistent in how it marks public domain. And Google is dropping support for license statements altogether. Etc.
Nemo