Hi Hans.
Yes... this would be easy enough to do. Of course you'd need to analyse the class and make sure there's exactly only one possible target @ProcessMethod, but that's easy.
I think it could also be possible to support automatic unmarshalling/marshalling by detecting that the target @ProcessMethod is arg is JAXB annotated. So, if the payload was XML and the method arg was JAXB annotated, you could automatically trigger JAXB to create the objects from the payload XML.
Then, for unmarshalling other payload types (non-XML), or for unmarshalling into objects that are not JAXB annotated, or for servicing @ProcessMethods with multiple args.... you could allow unmarshalling by defining a smooks binding config on the action.
Anyway... we'll see :)