After pulling my hair out for the last three days in my first non-upgraded ASP.NET 4.0 Web Application, I stumble across this post explaining it all.
http://jai-on-a
Whether the form is specified on the default page, or in the master page, it is still accessible via the Form property on the page.
Therefore, you should be able to set it's action in the code behind for that page.
protected void Page_Load(object sender, System.EventArgs e)
{
this.Form.Action = "Default.aspx";
}