I have an ASP.NET web form that is using JQuery on the client-side. I have a user interface that is building a collection of objects and storing them in JSON. With my client
JSON in the Hidden Field is a valid way to do it, as the data would then be posted to the server. You could then use the System.Web.Script.Serialization.JavaScriptSerializer component to deserialize the data (to a dictionary) and access the data that way. Not 100% sure hhow array data comes out of that process. THere are also other tools like JSON.NET too to parse JSON.
Another way is via a web service call, but that doesn't go through the page lifecycle.
HTH.