The ban on bypassing DRM only applies to copyright protection measures, and if a work is in the public domain there is no copyright protection on it. It does unambiguously affect fair use of copyrighted works, but just dumping fully-PD works that happen to reside on DRM-protected media ought to be OK.

It had been a while since I read the DMCA, and I was mistaken. It says "No person shall circumvent a technological measure that effectively controls access to a work protected under (copyright)" so it doesn't need an exemption.