How do you do that for IPv4... There's nothing new here. The failure modes are identical and your NAT box in IPv4 doesn't protect you from this any better.
With IPv4 I don't generally use two sets of prefixes for the same traffic from the same site to the Internet unless there is some sort of appliance in the path that somehow decides the "best" one to use for NAT and even then I am not convinced of such a device's utility in a general purpose sense. There might be corner cases where such an option is useful, though. I was making the point that trying to use two prefixes for the same traffic from the same site as some sort of redundancy doesn't really offer it because it only offers relief if your link to the provider drops. There are all sorts of other problems that can happen out on "the net" to make one prefix reachable but the other one not reachable from a remote site. Multihoming the same prefix from two providers is generally more reliable because if the remote network can "see" either provider, you are good and traffic can "fail over" from provider A to provider B in the course of a transaction without disruption. To recap, this tangent of the original thread was about the typical practice at small offices without a lot of network savvy to number the network in 1918 space and use a NAT at the Internet edge. To change providers, you simply change the NAT pool and you are done. With v6, while changing prefixes is easy for some gear, other gear is not so easy. If you number your entire network in Provider A's space, you might have more trouble renumbering into Provider B's space because now you have to change your DHCP ranges, probably visit printers, fax machines, wireless gateways, etc. and renumber those, etc. And some production boxes that you might have in the office data center are probably best left at a static IP address, particularly if they are fronted by a load balancer where their IP is manually configured. The complaint was that there is no equivalent in v6 and that someone is probably going to build and sell one and we will be right back in the same situation with v6 with networks in ULA space being NATed at the edge. People aren't going to want very much of their network infrastructure support tied to a provider's IP space. The small operation of which there are millions in this country, cannot justify the expense of multihoming for the sole reason of having an IP address range that doesn't change. As soon as the same configuration currently used is available for v6, you will see mass adoption of it. The lack of this currently in the market is probably one of the major drags on the adoption of v6 in the small office environment. People just do not want to number their internal network into PA space and can't justify the requirements to get PI space.
In fact, even multihomed BGP doesn't protect you from this unless you're taking a full table (which is a lot more practical in IPv6 than IPv4).
Owen