It's been a while, but I read through the OASIS spec for SAML for my implementation. There is a lot of yak shaving and I also had to implement XML Digital Signatures (XML-DS), and XML Canonicalization (C14N), but overall I don't think it took too long for something that worked, and was a library so I could just give it a key (or PKCS#11 handle) an identity and get back a SAML Assertion, which is fundamentally what I was after. It's available for viewing here [0].

[0] https://github.com/rkeene/saml-idp/blob/master/lib/saml/saml...