(idea came via jackson-users.ning.com discussion, suggested by Henrik Lundgren)
It would be nice to be able to do inverse of @JsonAnySetter – define a method that returns a Map (or possibly just collection of Map.Entry instances, or even just iterator?) and entries would be serialized as main-level attributes of enclosing object on serialization.
Although it would be possible to further generalize this idea (@JsonUnwrap?), for now it may be best value for development time to implement this in more limited form.