One could always try rewriting IPv6 into a ipv4 extension as rfc1726 hint that some IP versions could be done, and then use the original ipv4 part as backward compatibility data, pointing towards a ipv4-ipv6 gateway. On the upside, it would be cooperative with the current ipv6 work.

I do not know if such attempts has been made or considered, or if its would be easier than the current approach.