Is it possible to have a modified version of document-v20.mod for a
particular project? Where would I place it? I tried placing it in
<PROJECT-HOME>\src\documentation\resources\schema\dtd but it did not
work.

Basically all I need is to add the "onclick" attribute to hyperlinks:

<!-- ==================================================== -->
<!-- Hypertextual Links -->
<!-- ==================================================== -->
<!-- hyperlink (equivalent of <a ...>) -->
<!-- http://www.w3.org/TR/xhtml2/mod-hypertext.html#s_hypertextmodule -->
<!ELEMENT a (%link-content.mix;)*>
<!ATTLIST a
 onclick CDATA #IMPLIED
 %common.att;
 %link.att;
>