Yes, what you found is correct, currently it can be done only inside ClientLoad event. For future reference:
<GleamTech:DocumentViewer ID="documentViewer" runat="server"
Width="800"
Height="600"
Resizable="True"
ClientLoad="documentViewerLoad" />
<script type="text/javascript">
function documentViewerLoad(sender, e) {
var documentViewer = sender; //sender parameter will be the DocumentViewer instance
documentViewer.setCurrentPageNumber(myPage)
}
</script>