Opening access to FOP configuration?

As I'm getting closer and closer to support multiple font directories comes the problem of how to define them. It seems logical to extend current convention, passing several paths to the novelang.fonts.dir VM argument, separated by platform's path separator. On Un*x it would look like:

-Dnovelang.fonts.dirs=my-fonts:other/fonts

But the path separator highlights that font definition becomes system-dependant (it's a semicolon on Windows). And anyways defining the fonts in the command line is unlogical as fonts are part of the rendering. So I'm thinking on embedding font directories names as XSLT metadata (this idea was already mentioned). I explored the possibility to embed the whole FOP configuration itself, which is XML, also. But opening direct access to FOP configuration would let the opportunity to do weird things:

• Font cache configuration.
• Default page settings. This makes only sense when configuration is accessed by multiple XSLT.
• Title of the PDF document. The probable need to get this title from a source document (like the Book) would make this approach redundant.

On the other hand, stuff like hyphenation directories, ICC profiles and target resolution for bitmap images make sense. But one good reason to let Novelang keep hands on everything passed to FOP is to ensure that every directory is a subdirectory of current project, therefore preventing security threats. So we could have something like:

<xsl:stylesheet
    version="1.0"
    xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
    xmlns:fo="http://www.w3.org/1999/XSL/Format"
    xmlns:n="http://novelang.org/book-xml/1.0"
    xmlns:nmeta="http://novelang.org/meta-xsl/1.0"
>
  <nmeta:fop version="1.0" >
    <target-resolution>72</target-resolution>
    <renderers>
      <renderer mime="application/pdf" >
        <fonts>
          <directory recursive="true" >
            my/fonts
          </directory>
        </fonts>
        <output-profile>
          profiles/EuropeISOCoatedFOGRA27.icc
        </output-profile>
        <filterList>
          <value>null</value>
        </filterList>
        <filterList type="image" >
          <value>flate</value>
          <value>ascii-85</value>
        </filterList>
      </renderer>
    </renderers>
  </nmeta:fop>

    ...

</xsl:stylesheet>

This really looks like FOP configuration (see FOP documentation for details), but what's not shown is all forbidden stuff. So we end up with the best of the two worlds.