Is there a reason why exceptions that occur during the invoke application phase are handled by the ExceptionFilter rather than by an interceptor? I've subclassed org.jboss.seam.core.Exceptions and registered some of my own ExceptionHandlers to allow conditional redirects based on the viewId, which the MockFacesContext registered by the filter does not provide. I've also noticed that if an ExceptionHandler triggered during invoke application attempts a redirect to a page that includes page parameters you'll get a null pointer exeption because the MockApplication does not include a VariableResolver either.
By the way, I've added my own interceptor that delegates exception handling to the Exceptions component and it seems to be doing the trick so far. Any chance we could see something like that become one of the standard Seam Interceptors? That coupled with an API for registering custom ExceptionHandlers seems to me like it would very nice to have.