I believe that dynamic and cascading parameters are supported in Stored Processes in the SAS 9.2 Platform. Consult the documentation for more information on how to implement these kinds of parameters.
In EG, you can populate the initial parameter selection list from a CSV file or from a SAS dataset. However, this definition is static and will not change (in SAS 9.1.3) when the underlying data changes. I believe there are ways to update the metadata or using JSP pages to create dynamic lists. Consult the developer's documentation for SAS 9.1.3 Platform or work with Tech Support for the approach that best suits your installation and your skills.
I used the technique to refresh the metadata for the particular stored process, after my monthly job completes. Using unix script job to refresh metadata.
Since the data changes once a month, I chose to use this option.