Although that is not an direct answer to your question, but...
Have you considered using Forefront UAG for your scenario?
http://technet.microsoft.com/en-us/library/dd861391.aspx
Forefront Unified Access Gateway (UAG) integrates Remote Desktop Gateway (RD Gateway) to provide an application-level gateway for RDS services and applications. Previously, RDS was published by tunneling Remote Desktop Protocol (RDP) traffic from the endpoint to RDS servers using the Socket Forwarding component; tunneled traffic was not controlled or inspected, and client endpoints required installation of the Socket Forwarding endpoint component.