I ma using Newtonsofts\' Json.Net to serialize some and array of objects to json.
The objects have a common set of properties but also have Meta property which is a diction
You can do this by creating your own JsonConverter and then adding an attribute to the class you want to serialize
[JsonConverter(typeof(MyConverter))]
Example here -
http://www.lostechies.com/blogs/rhouston/archive/2008/02/25/a-custom-converter-for-json-net.aspx