Can you google about JAXB as I guess it have some options to control the output. Then we may have to add a way in the Camel JaxbDataFormat to configure those options as well.
The properties are set on the Marshaller as follows:
Marshaller mar = ...
I am pretty sure (correct me if I am wrong) that camel lazily creates the marshaller, or when the camel route is started, not allowing the developer to access the marshaller before the route is created.
Maybe to support setting JAXB properties (on lazy init) they could be passed via the camel route:
.setHeader(Marshaller.JAXB_FRAGMENT, constant(Boolean.True)) // don't add xml declaration