Pass custom data to FBA Membership Provider

Question

I need to have some additional data sent to each call to my membership provider. For example SPClaimsUtility.AuthenticateFormsUser takes a URI, a username and a password. The username and password are passed through to the Security Token Service (over WCF), and ultimately to my MembershipProvider implementation. I need to pass some additional context information to my MembershipProvider. One idea is to add a header to the OperationContext outgoing message headers and then inspect the incoming message headers in my Membership Provider. The problem is that there is not OperationContext available for me to send the headers before the call to AuthenticateFormsUser. Any help is appreciated!