Big news, I have a first working version of an UNO component to handle GStreamer from UNO code. This will allow using gstreamer features within UNO framework such as macro but also Java, c++ or pyUNO programs.
This component has many advantages for OpenOffice.org: it uses a free multimedia framework, and provide an API. This API has to be extended and doesn't give access to every GStreamer feature.
Of course this is still under development, and may not work in some cases. However, every feedback is welcome. Note that the component's page will be back soon. Many thanks to Laurent Godard who help me to debug and test the component