NAT66 implementations exist, and you don't need anyone's permission to use them. But I think it would be more productive to pursue smarter ways to share a /64, than to modify packet headers in flight.
For most purposes, ND proxy is the new NAT.
For most purposes, ND proxy is the new NAT.
I do think those uses will tend to involve 1-to-1 prefix translation rather than many-to-1 mapping.