I need some functionality offered by maildrop but lacking in procmail. It would not be difficult to configure my existing users to use maildrop, but those changes would of course be overwritten when ISPConfig rewrites user config files. I'm grepping through source files looking for places where the .procmailrc and .forward files get written, but I'd feel a lot more comfortable if someone who really knows the source code would point out all the places I need to make changes. I'd hate to get 80% of the changes made then have the remaining 20% bite me in the butt!
Please check out /root/ispconfig/scripts/lib/config.lib.php and /root/ispconfig/scripts/lib/classes/ispconfig_procmail.lib.php.