I use BIRT since early days and still have riddles regarding PDF emitter.
Short story: Can I configure fontsConfig.xml to load fonts from relative
With some BIRT versions, you can use a SystemProperty "birt.font.dirs". This overrides the fontsConfig.xml.
Well, this once worked, but obviously it was removed from the BIRT source code later.
Now you can call something like
EngineConfig engineConfig = new EngineConfig();
URL fontsConfigurationURL = new URL("file:///path/to/my/fontsConfig.xml");
engineConfig.setFontConfig(fontsConfigurationURL);
Platform.startup(engineConfig);
This allows you to supply BIRT with a customized version of fontsConfig.xml without poking around in the JARs.
Tested with BIRT 4.3.0.