The fact that you are using Struts for a web application would make very little difference when you're designing it. You design a Struts financial application in exactly the same way you would design a non-struts financial application. You'd define use cases, map the navigation flow, draw class diagrams, etc. It's only when you got down to the detailed technical design that you would start defining action mappings, ActionForms, etc.
This link from Sourceforge has a lot of good "real world" examples of Struts applications that you can download.