WCF SOAP custom headers for giving the computer name of the sender?

I have a WCF Web service that is consumed by client computers sitting behind firewall of LAN. So the service know only the IP address of the router shared by all these client computers. Now my boss want to know which computers made the request.

I am thinking of these possible solutions:

1. Http headers. However, some company's firewall might remove custom headers they don't like or recognize.
2. SOAP custom headers.
3. Make breaking changes to the service interfaces, and each function calls will contain a parameter for computer name of each client.

solutions 2 and 3 look technical feasible, however, I am not sure if SOAP custom header is the right place to document the technical details of the sender.

What would you suggest for identifying the computer sending the request?