You could implement a custom login page but it must be located relative to the root of the site, you would plug it in by adding a user.config setting like this:
<add key="LoginPageRelativeUrl" value="/yourloginpage.aspx"/>
where the first / must represent the root of the installation of mojoPortal
That setting will make the login link point to your url and should also make it redirect to your custom login page. However, the user must also exist in the mp_Users table or you will get errors after setting the auth cookie to log the user in.