Well, wherever it checks for immunity, e.g.
PHP Code:
if (access(targetid, ADMIN_IMMUNITY))
return PLUGIN_HANDLED
you can change it to
PHP Code:
if (access(targetid, ADMIN_IMMUNITY) && !access(adminid, ADMIN_RCON))
so it will only quit or print a "target is immune" message if the admin who uses the command does not have the ADMIN_RCON flag.