You could always run a database query, with the specified IP address, and return all user information from that IP. It could easily be created in the admin cp, for any coder willing to do it. I'd do it myself, but I'm rushed for time nowadays.
Basically, you just need to run:
PHP Code:
SELECT * FROM user WHERE ipaddress = *IPADDRESSHERE*
This is depending on the fact you have phpMyAdmin installed, or something similar. Alternatively, you can alter your config.php, and change the permissions to allow you to run a query.