Hello Kernigh,
first, I should repeat that we are working on a Free Content Definition at http://freecontentdefinition.org/ and a list of free content licenses at: http://freecontentdefinition.org/Licenses
It would be extremely unwise for Wikisource to categorically exclude other licenses than the GNU FDL, particularly since the FDL is in many ways a flawed license (overly complex, requires print-out of full license text together with printed works, has a vague DRM clause, is not adapted to different national legal systems, etc.).
Wikisource should instead follow similar principles as the Wikimedia Commons and allow any free content license to be used for added works. It is true that this will result in compatibility issues in some cases. However, the primary use of Wikisource is as a repository of free texts. As long as the individual texts allow the core freedoms of study, modification and distribution, Wikisource is servicing the community of content users.
The policy could make clear that all content contributed directly by the users, such as annotations and translations, must at least be dual-licensed under the GFDL. But excluding all non-GFDL compatible licenses strikes me as both counter to the philosophy and the goals of the project. It is not legally required, either, since the works in Wikisource are clearly separable.
I've forwarded your posting to foundation-l since this is a Wikimedia issue, in my opinion.
Erik