That is correct, I haven't checked this since Mx5 but as far as I know you can't subscribe to a single lognode.
I had to do the same thing for a project once. And I did this through a Java action that simply filtered on the lognode name. See this java file
I'm not entirely sure if this action is Mx6 compatible, I thought the messages and/or lognode name have changed slightly since I created the action.