Source IP in HTTP header when doing SNAT

I have kind of a strange request here, but the customer wants the original IP to be in the HTTP header for the application to read. However, we are doing SNAT on the LTMs, so we're not sure if this is even possible. We're doing global load balancing with LTMs in POPs around the world. So the GTM gets them to the closest LTM, then the "remote" LTM (in the POP) sends the traffic to the main datacenter in the US. We use SNAT between the POP LTM and Data center LTMs. The users' traffic is natted at the POP in order for the network to route it back properly to the appropriate POP they came from. The application sees the source as the POP LTM, but now they also want to see the original source IP as well. Is there an iRule that could accomplish this? Keep the original IP in the HTTP header even though the source IP is being natted?

 

 

Here's how the flow works:

 

 

User -> POP LTM -> Data center LTM -> server