[welterde]

What do you mean it has never been tried? That's how a lot of mobile providers and home internet providers operate today. My provider in Germany was already using DS-Lite (native IPv6 and IPv4 is tunneled over IPv6 to CGNAT gateway) more than 8 years ago. Mobile phones often use 464XLAT where IPv4 is translated to a region of the address space within IPv6 (the backbone is IPv6-only).

The problem is the inverse direction (IPv4 -> IPv6) since IPv4 is lacking any mechanism for forward-compatibility.