Looking into SCA more, I guess the Data Bindings would cater for what I have called 'default transformers' - so the switchyard transformers mechanism would be used in cases where a data binding implementation was not relevant?
I think we can accomplish everything we need with type converters and our transformer definitions based on message types defined in service contracts. The data bindings you are referring to are part of SDO, IINM, and I don't think we need to introduce SDO in order to get the behavior we want.
In the example you gave above, it would be pretty easy to introduce a JAXB transformer that can be independently configured with JAXB mappings or uses JAXB annotations directly on objects to be transformed. We should put together an example of this for 0.2 to see how it all hangs together.