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
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.