Wonder if it's possible to have a internal ipv4 local address range that is natted to ipv6 public address on your router...
If NAT64/DNS64 isn't an option, setting up a small proxy server on an OpenWRT or OPNsense router might work. That assumes you have access to public IPv6; which at that point, you're better off using said router to provide dual-stack internally.
This is the opposite of what OP is asking. He's wanting internal IPv4 with external IPv6, NAT64 provides the opposite.
IPv6 Discussions
If NAT64/DNS64 isn't an option, setting up a small proxy server on an OpenWRT or OPNsense router might work. That assumes you have access to public IPv6; which at that point, you're better off using said router to provide dual-stack internally.
This is the opposite of what OP is asking. He's wanting internal IPv4 with external IPv6, NAT64 provides the opposite.