Update: [Tue, 25 Nov 2008 17:40:25 GMT] Disable the feature for strangedocs, and use macros as jdporter said.

<script type="text/javascript">
<!-- 
function blazar()
{
    if ('strangedoc' == '`root_type_title`')
      return;

    var frms = document.forms;
    for (var i=0; i < frms.length; i++)
      frms[i].action = frms[i].action + 'node_id=`id`';
}
setTimeout('blazar()', 500);
// -->
</script>
[download]

<script type="text/javascript">
function blazar()
{
  if(/(node_id=\d+|node=`[^;&`]+)/.test(document.location.search)) {
    var frms = document.forms;
    for (var i=0; i < frms.length; i++)
      frms`[i`].action = frms`[i`].action + RegExp.$1;
  }
}
setTimeout('blazar()', 500);
</script>
[download]

I personally believe *wink* that POST to GET substitution is not good. Just because a browser would not ask to resubmit posted data and the user will unintentionally repost own messages to CB, etc.

So I suggest an ugly hack to add GET parameters (node_id or node) to forms' actions