There is No- membership attribute to meet your requirment.

You can convert Login Control to Template and try removing the UserName controls but I belive that is going to throw an exception.

So you can simply place a Label (Password), Textbox (txtPassword) and a Button (btnLogin).

Now in the btnLogin Click explicitly call Membership.ValidateUser("someUsername", txtPassword.Text)

and if it returns true set Expliclity FormsAuthentication Cookie by calling FormsAuthentication.SetAuthCookie("SomeUserName", false)