OK, regarding whitelisting, I finally found the solution and answered my own question:

Luci > network > DHCP and DNS > general settings.
The last box is “Domain whitelist”. Enter the captive portal domain here eg. captiveportal.com.
I believe this has the same effect as editing the /etc/config/dhcp file (under config dnsmasq) and adding the line:

list rebind_domain captiveportal.com

Regards,
Glitch

PS. @sammo: thanks, reading the link now.