You can use the new ClientPageChange event which is available since v2.7.0. This way you will get an event whenever a page changes, i.e. when clicked on thumbnail or when clicked up/down buttons in the toolbar:
<GleamTech:DocumentViewer ID="documentViewer" runat="server"
Document="SomeDocument.pdf"
ClientPageChange="documentViewerPageChange" />
<script type="text/javascript">
function documentViewerPageChange(sender, e) {
alert("Viewing page " + e.pageNumber);
}
</script>
For example, you can show a modal dialog which asks for a permission (blocking the DocumentViewer in the background) in this event.
Let me know if you still need a specific event only for thumbnail click, we can also add it.