The problem is ... telling real claims from false claims.
Is there anyway to legally establish a works public domain status other than a court case involving the publisher who is claiming copyright?
Yes you can copy the text ... put in on your website for free. Your ISP gets a DMCA letter ... your website is taken down immediately. As soon as they see it on sale at Amazon.com with a copyright notice ... well good luck getting them to let you put the content back up.
So then you hire a lawyer and sue then for abuse of the DMCA ... oh wait ... there is no penalty for making fake DMCA claims. OK you hire a lawyer and sue them for copyright fraud ... oh wait their is no legal penalty for falsely claiming copyright on a public domain work.
I am sure there is some way to legally establish a works public domain status ... but its going to take a lot of time and money.
There needs to be a penalties for false DMCA claims, for false copyright claims .. enough so that no company would knowingly make false claims.
There also needs to be a government project to certify works as public domain. People could submit works .. copyright holders could submit evidence ...