getElementsByTagName, CreateFakeImage

<r:link page="3">Page three</r:link>

var FCKInternalTagProcessor = FCKDocumentProcessor.AppendNew() ;
FCKInternalTagProcessor.ProcessDocument = function( document )
{
  var aLINKs = document.getElementsByTagName('R:LINK') ;

  alert(aLINKs.length);

  var eLINK ;
  var i = aLINKs.length - 1 ;
  while ( i >= 0 && ( eLINK = aLINKs[i--] ) )
  {
    var oFakeImage = FCKDocumentProcessor_CreateFakeImage( 'FCK__InternalLink', eLINK.cloneNode(true) ) ;

    eLINK.parentNode.insertBefore( oFakeImage, eLINK ) ;
    eLINK.parentNode.removeChild( eLINK ) ;
  }
}