I sent a survey on the preferred license model for papers and for metadata (mostly of bibliographic nature) to former editors of proceedings published at CEUR-WS.org. The results indicate that the preferred license model is CC0 (public domain). The votes so far are:
5: no change (i.e. no specific license)
30: CC0 (i.e. public domain)
The CC0 license is the simplest one. Note that this is only about the metadata, not the papers. So, basically the references to the papers can be used as public domain data. This allows organizations that build indexes of scientific output to use the index files of CEUR-WS.org volumes without further licenses and without asking the editors for permission.