The ideal solution would be for Nokia to actually ship the Vorbis decoding plugin from upstream. Or let kulve help them out with gstreamer?
I wonder if a strategy similar to how the Livna repository (3rd party non-free for Fedora) packages xine's non-free plugins might work: build the entire modified gstreamer framework (with a single .dsc), and then just exclude or remove the files already packaged by Nokia.