Workaround: Enable the accessibility full keyboard based interaction for handicapped users by pressing Command+F5, the system will switch from a list of user icons to display two text fields. It will beep once and you can then enter the user name, press tab and it will beep twice. Enter the password for the user account. If successful you will then be presented with the normal user account login screen. Press Command+F5 again to turn off the Accessibility features.
Warning: You must know the username and password for at least one user account in order to authenticate and decrypt the disk which will give you access to the standard login window.