The only reasonable way I can see to do this, would be to have an IPv4v2 packet within an IPv4v1 packet. So the v4v1 packet is delivered to its endpoint then s

The only reasonable way I can see to do this, would be to have an IPv4v2 packet within an IPv4v1 packet. So the v4v1 packet is delivered to its endpoint then stripped and the v4v2 packet traverses onwards to its destination. This doesn't solve the dual-stack issues, as either the endpoint needs both v4v1 and v4v2 stacks, or it needs to be offloaded at the router - essentially re-inventing NAT. What v6 has done, is essentially say there's no way to do this without breaking things, so we might as well break everything at once. So a lot of things that were optional, hacky bolt-ons in v4 have been adopted properly, because if we're going to add a 2nd stack, we might as well make the effort worth it - because lets face it, we're not going to have another chance to make a 20+ year transition any time soon.