I'm not completely convinced that this is such a good thing. Main idea behind PD is that you can do basically anything with PD work. Now suddenly PD works can become liability when used if not cleared specifically, which seems counter-intuitive.
If the work is yours, you can use DRM.
If the work isn't yours and it's copyrighted and you know who the author is, you ask permission to use the work, and permission to apply DRM.
If the work is public domain, you don't apply DRM.
The scenario you describe is works for which you don't know the copyright status. Your choice is to either go ahead and use the work without DRM, or apply DRM and potentially face consequences.
In this last case, the case you seem so concerned about, what purpose would DRM serve? It isn't your work, you should not care if anyone copies it.