A friend figured it out for me. Perhaps others can use it
In admincp/user.php
Find:
$cell[] = "<a href=\"usertools.php?$session[sessionurl]do=doips&depth=2&ipaddress=$user[ipaddress]\" target=\"_user\">$user[ipaddress]</a>";
Replace with:
$cell[] = "<a href=\"usertools.php?$session[sessionurl]do=doips&depth=2&ipaddress=$user[ipaddress]\" target=\"_user\">".gethostbyaddr($user[ipaddress])."</a>";
Extremely inadvisable. Many IPs, especially those of proxies and businesses, do not resolve, and for each attempt at resolving the IP, your page will load 5 to 10 seconds slower.