Hi

Should be possible, although I never did this.
Have a look at the sapphire/security folder. There you'll find the MemberLoginForm and MemberAuthenticator which are the classes used to login a member.
The Security class is the Controller that's being called when you visit yoursite.com/Security/login. It gets the registered authenticators and displays their login form(s).

You can create your own LoginForm/Authenticator (or inherit from MemberLoginForm/MemberAuthenticator) and register it as default authenticator (see Authenticator::set_default_authenticator).
This should be doable without altering core files, so you can place all your code in the mysite/code directory (maybe in a subfolder named "security").

I've just done this and documented it on my blog.