Marcel, lets keep this thread to discuss the seam openid/saml integration work that you have done.
1. Dependence on openid4java
Even though openid4java is the prominent oss library for openid integration, I have always wondered about direct dependence on openid4java API. Toward this, I had created a bunch of interfaces in the picketlink-web module to wrap the openid4java calls. Not sure whether this abstraction is the right thing to do but it certainly has the advantage of replacing the underlying implementation over time.
There is a JAXBUtil class in PicketLink that does smart caching of JAXBContext for performance. In your case, you use JAXB for configuration only and should not really matter.
3. package org.picketlink.identity.seam.federation overload
Quite a large number of classes in this package. Maybe you can move exceptions, factories to appropriate sub-packages.
Maybe next step would be to use javadocs to document the code.
There is JBossSAMLConstants class available in PL core.