What you describe there is basically NAT64 [1]. Not a ton of reason to actually deploy NAT64, though, when practically everything's going to be dual-stack anyways and you're not saving any IPv4 addresses versus that dual-stack arrangement with a typical IPv4 NAT. (There are also compatibility issues to consider; not everything can work with IPv6 addresses at the application level, especially early on in the transition.)

[1] https://en.wikipedia.org/wiki/NAT64