That's one ugly hack, which assumes (1) WAN1 has static ipv6 (the typical SME has dynamic DHCPv6 address...) (2) all the devices will behave correctly when running on NPT on failover WAN2. Many devices do not know about NPT which is basically NAT for ipv6, and break on p2p protocols like voice, video, streaming. They'll send the wrong NPT address to the other side, which try to connect back to the WAN1 address, which is down because of failover.