My company has a router/firewall deployed with the "common" Proxy ARP setup: it sits between some servers and the ISP gateway, and intercepts all the packets with proxy arp, so that it can apply both filtering rules and traffic shaping for enforcing bandwidth constraints. Using Proxy ARP means that, if the router/firewall breaks for any reason, we can simply pull it off the network, connect the ISP gateway directly to the server switch, and the service keeps going on (though not firewalled/shaped).
See: http://www.shorewall.net/ProxyARP.htm
Any idea how can we convert this setup to IPv6? I tried looking around but couldn't find any resource on the matter.
The page you linked to gives an answer:
If you're using Shorewall, then you're about 90% done already. Otherwise you'll have to configure your own NDP proxy, such as ndppd.