I'm interested in changing the standard JavaDoc Doclet to generate some additional documentation before the normal output of tags. Looking at the code (using a decompiler) I can see that my only real option is to download the source for HtmlDoclet and friends and make a few modifications ... but the only source that's available is for the 1.3 version of the code, which doesn't understand recent updates such as annotations and so forth.
Here you will find a hint to The Source for the Standard Doclet and a note that The source files are located in the directory src/share/classes/com/sun/tools/doclets. In fact, the subdirectory to consider might be langtools.
The linked JDK Source Code Release is a bit outdated. A actual JDK source code download can be done from here. If you download and install the
jdk-6u23-fcs-src-b05-jrl-12_nov_2010.jar file you will find what you are looking for at the location explained above.