Can you explain in more detail what you mean by "when a POST is made back from the application server"?
From your information I understand the following
* Node is an external server, hosted at the customer, reachable via Tunnel
* Pool is using tcp/80 or tcp/443 to communicate with the node?
* VS is on the external VLAN of the BIG-IP
* Virtual Address resolves to another FQDN, than the node itself. So the Host header is different and needs to be rewritten
Please confirm, if my understanding is correct.
However, possibly this is not really the point of your question. You're asking for a POST, that is sent by the nodes, correct? This would never hit the BIG-IP, as long as the nodes doesn't route the traffic to Salesforce through the BIG-IP. Even if they do, they will not be translated to the Virtual Address of your VS, as long as you don't configure static SNAT or NAT. Before describing solutions for this, I would like to know, if my understanding is correct, because there are several ways to solve this.