We'd like to have an overview document at a package level (below
the top level) that contains no classes.

In order to get my package.html doc to get assimilated into the
javadocs, I have to have a class in the package.  Otherwise, javadoc
seems to ignore the package.html (even though there are subpackages 
containing numerous classes).  To work around this, we add a 
placeholder class to the package:
 
  class Placeholder {
  }
  
Then as long as you don't specify -package or -private on the Javadoc
invocation line, it won't include the package scope class, but it will
include the package.html in that package directory.
 
What happens is we have a package that extends Swing functionality 
for our GUI API (AppWorks).  We have a package AWComponents that
are extensions to our AppWorks package:

Landmark.AWComponents.actions
Landmark.AWComponents.actions.edit

However there are no classes in the Landmark.AWComponents top level
package. But, we have a package.html in that package that
explains what the structure of the hierarchy is and the
purpose of the whole package.

so ... I put a PlaceHolder.java file in there and the package.html 
gets javadoc'd into the package-summary.html like a champ now. 
Again, so long as you don't -package or -private the javadoc, 
you never see the otherwise useless class.