The final feature to be included in the upcoming release are CD Stubs. From the documentation page:
Many times people would like to contribute CD data to MusicBrainz, but would prefer to not learn to be part of MusicBrainz. These people would rather just toss some data into a pile and go on with their lives to listen to their music or rip their CDs. CD Stubs enable users to do just that.
With this release people who come to look up a CD, but MusicBrainz does not know about that CD, will be given a choice to either enter the CD as a CD Stub or to join MusicBrainz and to enter the CD into MusicBrainz proper. (This is the red pill/blue pill decision point). Note that users who are logged in to MusicBrainz when looking up a CD will not be given the option to enter a CD Stub.
To submit a CD Stub to the test server, use any MusicBrainz enabled CD lookup application to submit a CD to MusicBrainz. You must not be logged into the test server, otherwise you will never be given the option to submit a CD Stub. Then follow the instructions for how to submit a CD Stub.
To retrieve the CD Stub you must use the XML Webservice. CD Stubs are not available via the old RDF web service. Use this URL to fetch data for CDs that will also return CD Stubs if no CDs are available in the main DB:
This URL is also available via libdiscid and its example program discid.