http://users.ox.ac.uk/~rahtz/passivetex/
Series of TeX macros
Developed by Sebastian Rahtz
Current version outputs to PDF or TeX.
Examples:
pdfxmltex FOinputfile
pdfxmltex manual.fo
You actually run the command twice on the same FO file to resolve all
references.
Installation can be a bit tricky, but output is regarded as high
quality.