<p>I've been looking around the System.Xml namespace, but don't see anything that would support this. Does anyone know if it's built into .Net, or would I have to obtain a third party library to do it?</p>

<p>NOTE: I wish it were as simple as generating  an xsd, but that won't do for my specific situation.</p>

I'm not sure about DTD but you can generate XSD files from an xml file using the
Xsd.exe
tool. It usually lives in the following directory:
C:\Program Files\Microsoft Visual Studio 8\SDK\v2.0\Bin\xsd.exe