How about setting Hidden="true" on the <ext:Tab>, the using .addTab() and .closeTab() in the TabPanel client-side API?
Example
<%@ Page Language="C#" %>
<%@ Register Assembly="Coolite.Ext.Web" Namespace="Coolite.Ext.Web" TagPrefix="ext" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head id="Head1" runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<ext:ScriptManager runat="server" />
<ext:TabPanel
ID="TabPanel1"
runat="server"
ActiveTabIndex="0"
Width="600"
Height="250"
Plain="true">
<Tabs>
<ext:Tab
ID="Tab1"
runat="server"
Title="Tab1"
Html="Tab1"
BodyStyle="padding: 6px;"
/>
<ext:Tab
ID="Tab2"
runat="server"
Title="Tab2"
Html="Tab2"
BodyStyle="padding: 6px;"
Hidden="true"
/>
<ext:Tab
ID="Tab3"
runat="server"
Title="Tab3"
Html="Tab3"
BodyStyle="padding: 6px;"
/>
</Tabs>
<Buttons>
<ext:Button runat="server" Text="Show Tab2">
<Listeners>
<Click Handler="TabPanel1.addTab(Tab2, 1)" />
</Listeners>
</ext:Button>
<ext:Button runat="server" Text="Hide Tab2">
<Listeners>
<Click Handler="TabPanel1.closeTab(Tab2)" />
</Listeners>
</ext:Button>
</Buttons>
</ext:TabPanel>
</form>
</body>
</html>
Hope this helps.