Ivo,
You could add the follwoing script to the index-default.html file in the head section:
<script language="javascript" type="text/javascript">
<!--
if (window.location != "http://localhost:8080/index-default.html"){
window.location="http://<your site name>";
}
// -->
</script>
Then when the index-default.html is accessed from your cloud site the window location would not be the one stated in the script and the user will be automatically redirected to the standard url and use OAuth. When you are accessing the index-default.html from the localhost you'll have the standard login.
Hope this helps in easy deployment of your app using OAuth.
Use login-default.html locally and remove this file from cloud.
Edit: You can change the login-default with the javascript you mentioned: (pseudocode) if not localhost then redirect
Ivo,
I use the option that Chris stated. Locally I have users for which I know the passwords, in the cloud the users have scrambled passwords in the app (30 char random). This way using the login-default.html will not allow users to access the application. Although this is a good method for non-production environments I personally remove the file when creating a deployment package or deploying directly to the cloud, as this is the most secure way of working.