Bumping this thread. This is indeed an issue. I guess I understand a bit why it isn't solved yet, since lots of packages seem to come directly from ubuntu's repositories.
Anyway, the script given above doesn't always work, in quodlibet it doesn't so I just patched its dbus plugin as shown in the first post for totem and sonata.
For quodlibet, it's /usr/share/pyshared/quodlibet/plugins/events/dbusmmkey.py
Patching banshee's /usr/lib/banshee/Extensions/Banshee.MultimediaKeys.dll doesn't work though.