v6 already implements a similar scheme. You set the protocol field in the v4 packet to 41, and then put the v6 addresses at the beginning of the packet payload,

v6 already implements a similar scheme. You set the protocol field in the v4 packet to 41, and then put the v6 addresses at the beginning of the packet payload, with the original packet payload following afterwards. It doesn't seem to have helped as much as you're claiming.