The tar archive includes two buildfiles with examples of how I'm using the PackagingType feature.
buildfile.pkgtypes - Creates what I called "api" packages, containing only interfaces / abstract classes. I made this mostly because I wanted a single project to produce a jar with public interfaces and another with the implementations.
Shows how to add funtionallity to predefined packaing-types like :jar.
I use api packages as dependencies on other projects.
buildfile.ejbdoclet - Create an "ejb" packagingType. This example shows a reusable XDocletPackage, currently I'm using it for ejbdoclet, but It may be easy to use it for other xdoclet needs.