What are you trying to achieve by authenticating by IP? If you are using it as a security feature, I would suggest your best bet would be to allow only specified IPs through your firewall, and use the normal extension authentication process in freeswitch.
The downside of course is that this would only be effective if you were blocking all other traffic, which you may or may not want to do depending on how other clients are configured; for instance, if some clients are connecting from a dynamic WAN address.