Let say an advanced home user or small corporation who didn't want to deal with PI space or BGP had two ISPs.
In the IPv4 world, this would typically be handled by selectively NATing the private IPv4s to either ISPs public IP addresses. For IPv6, from what I understand, you'd simply announce assigned prefixes from both ISPs to your LAN(s) using RD or DHCPv6, and each end node or application could pick the source IP address based on configuration (the application or ip addrlabel/prefixpolicy).
Question is, would the core or edge router need to do policy routing based on the source prefix to send it out the proper default route to the ISP, or is there a built in behavior in IPv6 which includes the source address in routing decisions? I presume the former, but I'm not sure.