said by squircle:IPv6CP is only involved with the link-local address and that's it; unlike IPv4 it's not even involved for the global unicast address which is the same as an IPv4 address. Address configuration is quite different with IPv4 vs IPv6 with PPP. If you take a look at Appendix A in that RFC you'll see that you have to use either RA or DHCPv6 to assign a global unicast address to the PPP/PPPoE interface and the only mechanism for propagating a prefix is DHCPv6-PD.
I'm sure there are unique challenges in automatically assigning IPv6 ranges; I don't know that DHCP/v6 is particularly well suited to PPP* connections, but I don't know if there's a way to do block assignment through IPv6CP (reading RFC 5072 ATM). I'd assume that doing block assignment through IPv6CP would be preferred and easier to implement (piggyback off the existing IPCP configs), but I won't speak for Gabe.