What about suuporting the XDG Base Directory Specification
http://standards.freedesktop.org/basedir-spec/basedir-spec-0.6.html

Right now
1) the registry is read/written under
g_build_filename (g_get_home_dir (),
        ".gstreamer-" GST_MAJORMINOR, "registry." HOST_CPU ".bin", NULL);

2) extra plugins come from
g_build_filename (g_get_home_dir (),
        ".gstreamer-" GST_MAJORMINOR, "plugins", NULL);

3) presets are keept under
g_build_filename (g_get_home_dir (),
        ".gstreamer-" GST_MAJORMINOR, "presets", NULL);

What about trying these first and have the older ones as a fallback (and fixme-0.11)
1)
g_build_filename (g_get_user_cache_dir (),
        "gstreamer-" GST_MAJORMINOR, "registry." HOST_CPU ".bin", NULL);

2)
g_build_filename (g_get_user_data_dir (),
        "gstreamer-" GST_MAJORMINOR, "plugins", NULL);

3)
g_build_filename (g_get_user_config_dir (),
        "gstreamer-" GST_MAJORMINOR, "presets", NULL);