Storing RelayState value from SP initiated SAML login

I'm looking for a way to store the RelayState value from SP initiated SAML login in a session variable. I have found articles that explains how to capture the RelayState value from the URI, however my SP send the RelayState and SAMLRequest using POST method. I'm thinking I could possibly use HTTP::Collect and HTTP::Payload and then run queries to capture the RelayState value, but I wonder if there is a simpler way of doing it, perhaps an existing iRule command (or tcl command) that already does this? I have also came across articles that mention the existence of SAML session variables (session.saml.last...) but i don't see them in the "current sessions" report. I use BIG-IP 11.6.0 Build 6.0.442 Hotfix HF6, so maybe these variables are only available in later releases.