If a project wishes to relicense its source code to a license that was incompatible with the original license, the project maintainers generally need to seek the express approval of its contributors if the contributors did not sign a Contributor License Agreement. If a contributor contributes code to a project under an anonymous alias, what does that mean for relicensing? Does contributing to a project anonymously mean the contributor loses copyright protection?
In this answer, I assume that someone has contributed code under a pseudonym (or under no name at all) and has not signed a contributor license agreement. Without delving too deeply into legal specifics (I'm not a lawyer), I will make a very simple point: ignorance of an author's identity does not diminish the author's rights under copyright, although it may severely complicate the author's attempts to assert those rights.
In all nations that are signatories to the Berne Convention, copyright protection is granted at the moment a work is fixed in a tangible medium. Therefore, whenever you encounter a work that contains copyrightable expression, you should assume it is already under copyright (assuming the copyright has not expired and the work does not predate the Berne Convention), and therefore it is your responsibility not to run afoul of that work's author's rights under copyright. If you need permission from the work's author to perform a particular action with that work, the onus rests on you to obtain permission from the author. Your inability to identify the author does not change your responsibility to respect that author's copyright.
To give a simple example, suppose you found a sheet of paper on the sidewalk with some code typed up on it. Surely, your inability to identify the author of this code does not mean the author forfeits copyright protection on the work.
Similarly, suppose someone slipped a piece of paper under your office door with some code on it, and attached a note saying it was released under the GNU GPL v3. Let's assume that the person who slipped it under your door really did have the right to license it under the GPL. (If they didn't, then you're quite a mess, but let's set aside this possibility and focus on the best case. Note that a good CLA would avoid this problem by forcing the contributor to bear the responsibility of any infringing material present in the contribution.) You can therefore reasonably use the work as permitted under the GPL, but you cannot relicense it without permission of the anonymous contributor, since that operation requires possession of copyright. If you attempt to distribute the code under a different license, you run afoul of the author's copyright.
There is, obviously, a serious practical obstacle to suing you for infringement, however: only the work's author can bring suit against an infringer. In order for a person to sue you for your unauthorized use of a work, that person must prove that he or she is the original author of the work. This may be a legal headache even in the best of times, and it may be all but impossible when the author has previously taken deliberate steps to obscure their connection to the work.
On a related note, the U.S. Copyright Office says this about pseudonyms:
An author of a copyrighted work can use a pseudonym or pen name. A work is pseudonymous if the author is identified on copies or phonorecords of the work by a fictitious name. Nicknames and other diminutive forms of legal names are not considered fictitious. Copyright does not protect pseudonyms or other names.
In no case should you omit the name of the copyright claimant [i.e., the declared copyright owner, when registering a copyright]. You can use a pseudonym for the claimant name. But be aware that if a copyright is held under a fictitious name, business dealings involving the copyrighted property may raise questions about its ownership. Consult an attorney for legal advice on this matter.
In short, a contributor's anonymity does not give you permission to use the contribution in ways that violate copyright, but it does complicate matters for a formerly-anonymous author who wishes to sue for infringement.