Issue 48811 (already written for the bow team, and scheduled for integration after 4.0 is 
branched) could solve this - you can make a tab "blink" a few times when you want to 
(works with sliding buttons as well).  The demo I used for testing it was to make the 
output window do exactly what you request.

However, there is a *much* simpler solution you could do right now, i.e.:

public class WrapperOutput implements OutputWriter {
   private OutputWriter realOutput;
   long timestamp = -1;
   public void println (String s) {
      if (System.currentTimeMillis() - timestamp > someNumberOfMilliseconds) {
        realOutput.printlin ("-----------------------------------------");
      }
      realOutput.println(s);
   }
  ...etc.
}

In other words, if you really want the output window to do this for you, I can't think of a 
way the output window could really help - there's no obvious time to *clear* the "*" or 
whatever that's appended, but you could easily do this just by printing some kind of 
divider if a long enough time has gone by since the last write.

Here's a very reasonable solution to this that requires no changes to the output
window:
 - Close the log output stream if more than n minutes go by with no writes
    - Tab will become non-bold
 - Write to the stream again when you need to
    - Stream will be reopened and tab/title will become bold again
    - If you want, find the output TC and call requestAttention() on it

I don't know what other way there would be to mark the tab - requestAttention()
is there for that purpose, and you can use the open state of the stream to
additionally indicate what you want.