Captive portal doesn't support IPv6 yet I'm afraid.
hi,
I know Captive Portal doesn't support IPv6 at this time, but in my opinion, "doesn't support" means make no side-effect on other things, it should not work on IPv6 and it also should not break currently well-worked modules.
However, I found a solution for this problem, just run
ipfw -q add 00001 allow ipv6 from any to any
in exec.php . This will pass all ipv6 traffic throw the IPFW.
I hope you can add this patch for Captive Portal