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
There is a way to have fonts on a relative path to be used in Birt.
What you need to do is copy the fonts to a location in your Web Application. Mine was in : C:\\src\main\webapp\Reports
Now in the application, use the following command to register the fonts from the above mentioned location.
FontFactory.registerDirectory( scContext.getRealPath("/Reports") );
This will register the font.
Tested on Birt 4.3 through Spring MVC