I have one which I can experiment with, but please be more precise what you mean by port-to-port?
Do you mean serve an incoming ethernet (or do you mean wireless?) TCP connection to TCP port X (or is it for UDP packets?) on the WAN port with server connected to a switched physical port listening on same TCP port X?
DD-WRT offers port forwarding and also port range forwarding under NAT/QOS tab. I have never had the need to do this and I don't have a bone-fide server running at the moment (just workstations) to definitively test this, but this is a DD-WRT feature.
Has anyone else tried this?
You might want to ask on a hardware specific forum since I think you need to know if the DIR-615 hardware supports this.