It seems I misunderstood your request, as I thought you were referring to my CONTEXT and not my REQUEST. The context I described (500+ definitions) is unavoidable in our project (transitioning from legacy code, and having very specific functional requirements), and so that context cannot change. The preparer we are using on all these tiles is required to add a kind of smart logic to determine how to apply (or not) a tile attribute's default value for each request (thus avoiding the need to add that logic code to the underlying JSP files).
So, my request for enhancement is really only about adding a way of specifying "global" preparers within a definitions file. Much like it is possible to define default interceptors and global results in Struts2.