Well, regarding my prior post, it works:

<asp:textbox ID="username" runat="server"/>
<asp:textbox ID="password" runat="server"/>
<asp:Button PostBackUrl="https://mysecuredomain/tester.aspx" Text="submit" runat="server" />

You can't access PreviousPage on the new server, but you can access request.form, and for some reason that escapes me, my textboxes transmit their values as variables named what the textbox control are named:

username=whatyoutyped&password=whatyoutyped

As long as it was a POST and the destination was HTTPS, then the data was transmitted securely.

So stick that in your pipe and smoke it.