I would really like to see gstreamer in Emacs, but I am not sure you can truly
prevent gstreamer from loading proprietary codecs.
This is primarily true if the code uses the playbin module for gstreamer, which most people do, and Po Lu is explicitly NOT doing. The manual process they intend will instead miss some codecs (etc) that are supported on the system, but it won't unknowingly load proprietary code (well, I guess modulo some extreme behavior on the system maintainer's part).
Hope that helps,