Using a default document located in a subfolder is easy, just fill in the path to the document, and it will be used (The path and document name will not be displayed in the browser).

<system.webServer>
  <defaultDocument enabled="true">
    <files>
      <clear/>
      <add value="path/to/page.ashx"/>
    </files>      
  </defaultDocument>
</system.webServer>