In order to achieve your scenario you could use a custom attribute on the RadAjaxPanel
as shown in the example below. Note that there is no build in mechanism to handle such cases and custom implementation is required either using a ASP.NET HiddenField as you have described or using custom attributes.
Regarding the question about the ajax operations: The idea behind the ASP.NET Ajax is performing ajax request by sending the page data to the server and executing the whole page life cycle but returning only the html parts that have been wrapper by an UpdatePanel.
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