Looks like Ubuntu Saucy needs XDG_RUNTIME_DIR (see 
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html) set.

This is part of the normal user session setup and needs to be duplicated
manually when starting a custom D-Bus session.

For now, you can do:
export XDG_RUNTIME_DIR=`mktemp -d`
before invoking dbus-session.sh. The directory can be removed when the session
ends.

Eventually I'll probably have to add that to dbus-session.sh. I wonder whether
that explains some of the random problems that I have seen with parallel
testing on Saucy and Ubuntu: multiple different D-Bus sessions probably ended
up using the same /run/user dir.