The STIX project is apparently about to release a set of OpenType fonts, progress can be monitored at http://www.stixfonts.org/finalsteps.html Depending on license conditions, these fonts could be used for testing FOP, either bundled with it or by asking users to download them separately.
Here is the user license for the STIX fonts: http://www.stixfonts.org/user_license.html
I just had a look at http://www.stixfonts.org/finalsteps.html and it still says: "The glyphs will be packaged for beta release into large OpenType fonts. [The following packaging is currently under review. Revised packaging will be issued as soon as a final decision is made.]"
resetting P2 open bugs to P3 pending further review