Thank you for the suggestion! We appreciate it.
The RadXmlHttpPanel control, as its name suggests is designed to receive HTML content from the server, and paste it on the client. Returning JSON is out of its scope, and implementing it would require major redesign of the control itself. That's why we don't plan to extend the RadXmlHttpPanel to support returning JSON, but we will consider this feature request when introducing new controls in our RadControls suite.
the Telerik team