I believe the latest version of Macromedia's Dreamweaver may have better support for custom tags, including the Struts JSP tags, but I haven't had a chance to play with it much myself.
Often, you only really need to stub-out the ActionForms to go with the JSPs. An action mapping that just forwards from an input page to an output page is often sufficient for a preview. It can also be helpful to setup a mock Action that posts some preview data in the request, which you can then display on the JSP for
testing.
Some people also find that page design can be easier with Velocity templages, since they are more forgiving than JSPs. Chapter 17 of Stuts in Action shows how to migrate a small application from JSP to Velocity
HTH, Ted.