Comment by Dagger2

It would be easy to make such an extension, but you're going to hit the same problem v6 did: no v4 stacks use your extension.

How will you fix that? By gradually reinventing v6, one constraint at a time. You're trying to extend v4, so you can't avoid hitting all of the same limits v6 did when it tried to do the same thing. In the end you'll produce something that's exactly as hard to deploy as 6to4 is, but v6 already did 6to4 so you achieved nothing.