How about making it possible for us (via customise active convos) to pin a limited number of threads ourselves? And Mumsnet Towers would only pin threads that you considered of general importance, without necessarily allowing us to be able to unpin those threads, so that important info could be broadcast.
Also, could we have a line under, or box around, pinned threads? Just to visualy separate them so that it is easier to keep an eye on what's happening at the real top of active convos.