There are one section 1.2 Installing/Building the Plugins on Ubuntu in Linux_Multimedia_Framework_User_Guide.pdf, which described how to build the gstreamer plugins on Ubuntu Lucid rootfs but it is only for native build and only for multimedia level. The BSP level packages are generated from rpm package.
The gstreamer related debian packages for Linaro 11.05 are located in one private PPA. I am not sure how to distribute it, I will check it. Anyway all the gstreamer related packages should can be build in linaro rootfs, but for better performance, one patch for gstreamer base package should be applied ( because of different gstreamer core version).