This only applies to SharePoint 2010.
What you need is a SAML token that SP can consume, so I believe you have to write an Identity Provider (IP) to plug into the SharePoint STS (Security Token Service). In that IP, you will implement the Authenticate(..) interface using code like this:
return AuthenticationResult.NotAuthenticated; // Caller will then 302 to Login page
Here's an overview of the process:
The programming details are hard to come by, but there are a few articles online and the book "Programming Windows Identity Foundation" is excellent. I've been meaning to do an article on this subject, but have not yet, otherwise I would just send you the code.