For ip6tables the option "--icmp-type" is called "--icmpv6-type"
A call of sed to replace any occurences of 'icmp-type' by 'icmpv6-type' worked fine to me.
by the way:
Line 123 in setup.cgi: the icmp-type "source-quench" is not awailable in ipv6. I used "package-too-big" instead.
sorry! "packet-too-big"
Thanks - these will be fixed in the next Webmin release.