Found it in the KnowledgeBase!

http://seamframework.org/Documentation/RetrievingServletParametersUsingEL

System-wide configuration like this would seem a handy way to configure universal differences between dev/test/prod environments common to all hosted applications.  (For example url's to environment-specific services like static content servers.)  There is the -ds.xml design for data sources and it would've been wonderful if that mechanism was generic for other uses, but using config-params is just as simple.