I just answered your question in the support ticket you opened. I am also posting it here. If you have any further questions, please place them in the ticket, so that we avoid duplicate posts.
Here is how you can make it work. I did not notice differences with the behavior of this approach in different RadControls versions, so it should work for you too.
1) Attach the onsubmit event, as you suggested - through the ScriptManager
2) Inside it, set a minimal timeout, so that the request can be processed
3) Check if it is an ajax request through the get_isInAsyncPostBack() property of the PageRequestManager
4) If not, show the div
5) (optional) You could use a RadAjaxLoadingPanel and show it explicitly over the desired element in the page
For your convenience I am attaching a sample project demonstrating this approach (I did not add the Skins dll to keep the archive as small as possible but runnable at the same time).
the Telerik team
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