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In 1 query I was just able to find a mashup site with over 1.9 million indexed pages, all 100% copied from everywhere else online including hotlinked images.
There are already methods of handling copyright infringement. You can file a DMCA request and the offending webpage will be removed from the serps.
The problem with creating a new tag is that many people won't adopt it.
Even if everyone starts using this brand new tag, what is to stop the offending site from applying a copyright tag to their page? The search engine is going to see you both have copyright meta tags and we are back to where we are now.