MenuItem attributes lost on ajax/postback

I have radcontextmenu where some items have custom attribute "ConfirmationMessage" with a value like "Are you sure you want to delete?"  These attributes are set on the server code-behind on load.

On the client in javascript - the OnClientItemClicking event checks that attribute and presents a radconfirm first. 

It works on the first load of the page, but the attributes are lost after ajax/postback. 

Please advise a fix or workaround.  I am already using the Text, Value properties.  Is there some clever way to use the NavigateURL field without causing it to try to navigate to a page?

Thanks

Sean