> There's no way around it: a non-IPng-having node will have to go through a translation box of some kind.

Yes. Note that it doesn't need to be someone else's relay; anyone with IPv4 connectivity could easily route 2002::/16 into IPv4-land (without having to announce it in BGP for others to use). You could even announce 2002:aabb:ccdd::/48 as a more-specific in BGP if you wanted, although this was more exotic.