Yubikey

For the server case, you would need to have a non-Yubikey login method not using two factor, and this applies to almost any two factor system I would think. Since this 'recovery login' would only be used in the 'something very broken' case, it's not too vulnerable to keyloggers on the client. Google Authenticator has a recovery process with pre-printed emergency token codes, which may be better.

For multiseat, a USB-based login method may not be very suitable as it requires the login process to know more about Yubikey - perhaps a smartphone or traditional token would work better.