Create a simple javascript script, register it on the page and call it when you want the page to open, rough example:
this.myNewWindowOpener.Target = "_blank";
this.myNewWindowOpener.NavigateUrl = "BLOCKED SCRIPTopenNewWindow('http://example.com/page.aspx)";
Page.ClientScript.RegisterStartupScript(this.GetType(),"windowopen", @"<script>function openNewWindow(target) {window.open (target,'NewWindow'); return false;}</script>",true);
Paul Leigh