On a recent trip to Korea, a customer asked a great question about how to implement a metadata-driven custom DataAction that needed access to a "Metadata" or "System" data control on each request. They weren't keen on manually registering the SystemDataControl in each application's DataBindings.cpx file, so we came up with an example solution based on a servlet filter that insures the "System" data control is registered dynamically in the binding context.
See example #48 in my list of not-yet-documented sample applications.
http://radio.weblogs.com/0118231/2005/08/26.html#a588