This is a bookmarklet for editing the value of inputs marked with the readonly attribute.

editReadOnlyInputs.bookmarklet.js

javascript:(function(){!function(){"use strict";function a(a){return a.attributes.readonly}function b(a){a.addEventListener("click",c)}function c(a){a.target.value=prompt("enter new value")}var d=document.getElementsByTagName("input"),e=Array.prototype.slice.call(d),f=e.filter(a);f.forEach(b)}();})();

editReadOnlyInputs.js

(function(){
  'use strict';

  var inputs = document.getElementsByTagName('input')
  var arrInputs = Array.prototype.slice.call( inputs );
  var filteredInputs = arrInputs.filter( getReadOnly );

  filteredInputs.forEach( assignEventHandler );

  function getReadOnly( input ) {
    return input.attributes.readonly;
  }

  function assignEventHandler( input ) {
    input.addEventListener( 'click', getNewValue );
  }

  function getNewValue( e ) {
    e.target.value = prompt( 'enter new value' );
  }
})();

To use, copy the editReadOnlyInputs.bookmarklet.js content into a new bookmark. On the page you would like to edit input fields with a readonly attribute set, just click the bookmark and then click on the field and you will be presented with a prompt for the value you would like to set.