What's the intention for fop.xconf?
It's been there for ages with the intent to provide for configurable, easily changed defaults. Unfortunately 1. There's not much more than PDF filters (in the maintenance branch), and if all filters are deleted, the code uses a flate filter anyway (which means you have to provide a nop filter in order to have a look at the uncompressed PDF code). 2. The fop.xconf, userconfig and command line options are not merged, although they should.