Experimente este simples UserScript:
// ==UserScript==
// @name OpenID fill out for StackExchange sites
// @include http://superuser.com/users/login*
// @include http://stackoverflow.com/users/login*
// @include http://serverfault.com/users/login*
// @include http://*.stackexchange.com/users/login*
// ==/UserScript==
(function ()
{
window.addEventListener('load', function()
{
document.getElementById('openid_identifier').value = "https://your.url/for/openid";
document.getElementById('show-more-options').style = '';
}, false);
})();