You can do this reliably with some JavaScript:
.
.
.
<head runat="server">
<title></title>
<script type="text/javascript">
function fileManagerLoaded(sender, e) {
var fileManager = sender;
fileManager.insert(0, {
xtype: "component",
region: "north",
contentEl: "pageHeader"
});
}
</script>
</head>
.
.
.
<div id="pageHeader">My custom text...</div>
<GleamTech:FileManager ID="fileManager" runat="server" ClientLoaded="fileManagerLoaded" />