Going back to the subject of protecting the CP against abuse, I noticed the "Maximum concurrent connections per client IP address" ($maxprocperip) setting. A quick look at the source code (captiveportal.inc and system.inc) suggests it sets lighttpd's evasive.max-conns-per-ip directive.
However, if the $maxprocperip "Maximum concurrent connections" field is left empty in webGUI config, it doesn't create a lighty evasive.* directive at all in /var/etc/lighty-Captive*.
I can't find any other way to enforce the "Default 4 connections per client IP, with a max of 16" so it seems like a small bug to me (either fix the comment in the webGUI, or put a value in $captive_portal_mod_evasive in system.inc)