Gerard Meijssen wrote:
... If this is correct, the consequence would be that material that is not available under the GFDL should not be included in a WMF project. ...
No. Assuming that "a WMF project" = any Wikimedia Foundation project, there are ways to insert non-GFDL content into such projects, and not only non-GFDL images (like CC-BY-SA) at Commons, en.Wikibooks, or elsewhere.
Read http://en.wikisource.org/wiki/Wikisource:Copyright
"If you contribute source materials written by someone else to Wikisource you thereby warrant that the source material is either public domain in the United States or that you have permission from the copyright holder to place the work on Wikisource."
I possibly misunderstood it, by I think that if I have some noncommercial or no-derivates license (such as CC-BY-NC-ND), then I am allowed to upload that source text to en.Wikisource.
-- [[Wikibooks:en:User:Kernigh]]