This is already available.  I have been using since I first upgraded from 6.0 to 6.1 to locate outdated clients for upgrade.

Create a new Session Group and set the Session Filter to:

GuestClientVersion <> $SERVERVERSION AND GuestConnectedCount > 0

This is an old one, with requests on the Feature Requests forum from 3 years ago. In that thread there was indication from a couple of years ago that the SC guys were looking into it but nothing ever got added.

AuthAnvil has a simple API that requires only two pieces of configuration: the URI to the service and the SiteID. A simple SOAP call with username and code (PIN plus OTP, entered by the user) returns a true/false value in the SOAP response. There's a PHP example in the documentation.

Can we please get this one implemented? I have an on-prem AuthAnvil server and a number of techs accessing SC from both here and in the field.