OK, just for you: Put the following code in your Free Nodelet:

<!-- the HTML: -->
Hide old replies: <input name="hide_old_replies" id="hide_old_replies"
+ type="checkbox" onClick="toggle_old_replies()" />

<!-- the JS: -->
<script language="javascript"><!--

    function toggle_old_replies(){
      var chk = document.getElementById('hide_old_replies');
      var state = chk.checked ? 'none' : 'table-cell';
      var allelems = document.all ? document.all : document.getElement
+sByTagName("*");
      var inc = 0;
      for ( i = 0; i < allelems.length; i++ ) {
        if ( allelems[i].className == "reply-body" ) {
          allelems[i].style.display = state;
        }
      }
    }

//--></script>
[download]

This only toggles the visibility of the bodies of "old" replies; the header boxes etc. for old replies remain visible. Hopefully that's sufficient for your desires.

Thanks to tye for applying the patch I wrote to make this possible.