I think we should automatically try to launch a user dbus session if dbus is installed.

Something like:
...
# add dbus-launch if found
dbuslaunch="`which dbus-launch 2>/dev/null`"
if [ -n "$dbuslaunch" ] && [ -x "$dbuslaunch" ]; then
command="$dbuslaunch --sh-syntax --exit-with-session $command"
fi
...

In /opt/gnome/etc/gdm/Xsession just before

echo "$0: Setup done, will execute: $command"
eval exec $command

...should do.