I agree with @krazeh you are exposing your LAN so that could be dangerous.
I propose a different direction to solve this: one-to-one nat:
One-to-one NAT (aka Static NAT) is a way to make systems behind a firewall and configured with private IP addresses appear to have public IP addresses.
Not sure how to do that with OpenWRT but basically you add all Public IP addresses to the WAN interface, then DNAT a specific Public IP address to the server in the LAN, SNAT traffic coming from the LAN server back to that specific public IP and allow FORWARD traffic to that LAN server.
Edit found some solutions but there are more: