Gathering resources...

I made a guess the JDT project was where the action would be.
Downloads - Eclipse Project JDT Downloads - Stream Nightly Build - eclipse-sourceBuild-srcIncluded-.zip where is a big string of characters that has the date and some kind of index number in case they ever do more than one of these in a day.

Download is pretty big and takes about ten minutes to my laptop over my wireless at home.

I expanded it out into a new directory on my hard drive I called "eclipse-source."

A little digging in the source reveals that templates are part of the JDT UI project.

(Of course, I found them the first time, but couldn't remember how to get back to them the second time. Windows search with "template*.java" reveals the following folders have related content:)
plugins\org.eclipse.ui.workbench.texteditor\src\org\eclipse\ui\texteditor\templates
plugins\org.eclipse.jface.text\src\org\eclipse\jface\text\templates
plugins\org.eclipse.jface.text\src\org\eclipse\jface\text\templates\persistence
plugins\org.eclipse.text\src\org\eclipse\jface\text\templates
plugins\org.eclipse.jdt.ui\ui\org\eclipse\jdt\internal\ui\preferences
plugins\org.eclipse.jdt.ui\ui\org\eclipse\jdt\internal\ui\text\template\contentassist
plugins\org.eclipse.jdt.ui\ui\org\eclipse\jdt\internal\ui\text\template\preferences
plugins\org.eclipse.jdt.ui\core extension\org\eclipse\jdt\internal\corext\template\java
plugins\org.eclipse.pde.ui\src\org\eclipse\pde\ui\templates
plugins\org.eclipse.pde.ui\src\org\eclipse\pde\internal\ui\wizards\plugin
plugins\org.eclipse.pde.ui\src\org\eclipse\pde\internal\ui\wizards\templates
plugins\org.eclipse.team.cvs.core\src\org\eclipse\team\internal\ccvs\core\client

Daunted yet?
Turns out the file:
plugins\org.eclipse.ui.workbench.texteditor\src\org\eclipse\ui\texteditor\templates\package.html
..has a decent introduction

So it was the workbench, not the JDT.

instructions.html in the root directory has build instructions for the source.

Must register my intent to make my noble improvements...
Where to look - newsgroups? bug list? e-mail list?

Article on user editing of eclipse templates:
http://www.javalobby.org/forums/thread.jspa?threadID=15796&tstart=30