That's similar to, but not exactly what we were discussing. In particular 6in4 has a full ipv6 header after the ipv4 header, but here the suggestion was instead that supplementary infomation would follow. For example, the most significant address bits could be stored in the ipv4 header and the least significant ones in the new part.

That's not meaningfully different. It would just amount to a slightly less redundant representation of the same data -- the steps needed to deploy it would be the same, and you'd still have all the same issues of v4 hosts not understanding your format.