Handler.cs

public class PostHandler
{
    ...
    public AjaxContinuation Execute(Model request)
    {
        var updatedModel = Map(_addWidgetService.Add(Map(request)));
        // How do I get my updatedModel back to the client?
        return AjaxContinuation.Successful(); 
    }
}

// Ah, maybe this is what I'm missing; adding the response data to the continuation:

var response = AjaxContinuation.Successful();
response.Data["data"] = updatedModel;
return response