Can't answer your main question, but from what I recall, DHCPv6 is kind of necessary anyway. It's the easiest/main way for the client to receive DNS server information, even if default gateway discovery is now down through multicast.
I believe there is another way, but the router has to support it and I forget what it's called.
I believe there is another way, but the router has to support it and I forget what it's called.