Hi,
I can tell you that Lotus Notes supports SSO using x.509 certificates, but in a proprietary way. I know they didn't support SAML, but they might have added it recently. Unlikely but possible. I have worked with many customers who have been wanting SSO with Lotus Notes, and they have had to get help from IBM to make it work. I suggest you talk with IBM to find out how to configure it.
Thanks,
Tim