Proxy-ARP can be very useful feature in many different scenarios. Considering that IPv6 does not use ARP for layer 2 address resolution, similar feature should be available for IPv6 too.
Proxy-ndp protocol is described in RFC4389 http://tools.ietf.org/html/rfc4389. It is also implemented natively in Linux kernel (net.ipv6.conf.all.proxy_ndp sysctl).
Typical problem that is solved with the specified feature:
http://serverfault.com/questions/165203/ipv6-routing-problem
I propose the NDP menu to be implemented similarly to ARP menu on the Ethernet interfaces, for consistency reasons. Alternatively, the feature could be implemented in IPv6->ND panel.