The function onPageComplete() is called whenever the last page is overflowed by a longer source. We first check that indeed event.contentOverflow is true (Try it):

function onPageComplete() {
  if (event.contentOverflow) {
    document.all("layoutrect" + lastPage).onlayoutcomplete = null;
    newHTML  = "<IE:DEVICERECT ID='devicerect" + (lastPage + 1) +  
               "' MEDIA='print' CLASS='masterstyle'>";
    newHTML += "<IE:LAYOUTRECT ID='layoutrect" + (lastPage + 1) + 
               "' ONLAYOUTCOMPLETE='onPageComplete()' NEXTRECT='layoutrect" + 
               (lastPage + 2) + "'  CLASS='contentstyle'/>";
    newHTML += "</IE:DEVICERECT>";

    devicecontainer.insertAdjacentHTML("beforeEnd", newHTML);
    lastPage++;
  }
}

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