Using container objects instead of fields ?

You know the POJOS, the one with getters/setters ?

 public class UserPojo {  
   private Serializable id;  
   private String login;  
   public Serializable getId() {  
     return id;  
   }  
   public void setId(Serializable id) {  
     this.id = id;  
   }  
   public String getLogin() {  
     return login;  
   }  
   public void setLogin(String login) {  
     this.login = login;  
   }  
...
 }  

What now if we need to serialize this to XML or JSON. Instead of using something like Xstream maybe we could do it like this :

 public class UserPojo {  
   private JSONObject jsonObject;  
   public UserPojo() {  
     jsonObject = JSONObjectHelper.newJSONObject();  
   }  
   public Serializable getId() {  
     return (Serializable) JSONObjectHelper.get(jsonObject, "id");  
   }  
   public void setId(Serializable id) {  
     JSONObjectHelper.put(jsonObject,"id",id);  
   }  
 ...  
 }  

We are building the JSON object as setters are called. No need of a reflection api . Although now we are dependent to JSON or XML or whatever... I could think of runtime proxy generation code that would replace the getter/setter code but I really don't like class definitions that hang in memmory. May be some spi stuff and a more general api could be introduced which would let your POJOs store the data in whatever format you choose.
As for me I think this is a good fit to use with MongoDB because know I can convert my POJOs easily  to MongoDB's DBObject:

 (DBObject) JSON.parse(hasJSONObject.getJSONObject().toString());  

and fill up my POJOs with MongoDB data:

 User user = new User();  
 user.setJSONObject(dbobj.toString());   

And again a good fit for JAX-RS  in which case It would convert my POJOs without going through some reflection process.
It would even be better if I could configure my POJOs with an api to store data both within a JSONObject and a MongoDBs DBObject without a compile time dependency.
Looks like a simple way to do things ?