@Marco: Do you know which conversions are provided by FasterXML: arbitrary XML → YAML, arbitrary YAML → XML, or both? If the underlying conversion rules are promising, we could include them a 'fasterxml' format in BaseX (similar to the 'jsonml' format).
As far as I can understand from the docs and the code on github there Jackson is a sort of hub with several dataformat modules which are able to parse and serialize from the various formats.
I don't know whether, with the term arbitrary, you mean to be able to provide your own syntax for the XML serialization. In this case I am not sure.
M.