They should offer NAT64/DNS64 gateway. It would work like other gateways and give IPv4 access to IPv6 hosts. Would probably be expensive like the NAT Gateway.
The other option are application-level proxies/load balancers etc., but that's not easy if you want to support all possible protocols.
This still requires mapping tables similar to explicit port forwarding, and also does not help with e.g. a scenario where all hosts need a specific port (e.g. 443) to be v4 reachable, right?
Network load balancers (NLB) are for other protocols.