Can SAML, and OpenSAML, only be used for web-based applications?
No. While SAML 1.0 and 1.1 were very web-browser centric SAML 2.0 is not (though it still contains some web-browser specific functions). Currently though, only web-browser and SOAP-service based profiles are publicly available and in wide use. For more information on the SOAP-based services refer to the Liberty Alliance work.