The project has released a replacement for wikibook/xml/tblatex.xsl which recognises the output of (:div class=block:) markup and prints the contents of the block in a different font family. If the main body is serif, it uses sans, and vice versa. It can be downloaded from Install the Server#toc3.
Sites running wikibook server version 1.0.11 (released Friday, 1 May 2009) can just replace the existing tblatex.xsl file with the new version. It is of course always a good idea to make a backup copy of the existing tblatex.xsl file.
For an example, see Text Blocks in a Different Font Famly. Thanks to Vito for suggesting this change.