You may wonder why we have not attempted to support what you are looking for. Essentially it comes down to the fact that the W3C has never clearly defined this. Please see the following:
DOM 1 (1998):
http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/level-one-core.html#ID-412266927DOM 2 (2000):
http://www.w3.org/TR/DOM-Level-2-Core/core.html#ID-412266927DOM 3 (2004):
http://www.w3.org/TR/DOM-Level-3-Core/core.html#ID-412266927(same for all links above)
The DocumentType interface in the DOM Core provides an interface to the list of entities that are defined for the document, and little else because the effect of namespaces and the various XML schema efforts on DTD representation are not clearly understood as of this writing.