The desired functionality could be achieved on the client. The idea is to replace "Is Null"
to "Is Absent"
in the preview element innerHTML
preview = $telerik.getElementByClassName($get(
"<%= RadFilter1.ClientID %>"
preview.innerHTML = preview.innerHTML.replace(/Is Null/g,
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed