Hat tip to Heri for the copy-maven-plugin tip, but, since it isn't immediately obvious how to set this up (even after reading the documentation), I'll post the actual answer here.
I found this example by digging through the GitHub issue tracker, and generally fighting with it.
Note that this code doesn't work on Maven 3.1.0 because of the Sonatype aether change that is documented in several SO questions. The author of this plugin claims (as of August '13) that the next release (version 3.0 apparently) will support Maven 3.1.0
Anyway, the plugin should be configured as follows:
<plugin>
<groupId>com.github.goldin</groupId>
<artifactId>copy-maven-plugin</artifactId>
<version>0.2.5</version>
<executions>
<execution>
<id>create-archive</id>
<phase>package</phase>
<goals>
<goal>copy</goal>
</goals>
<configuration>
<resources>
<resource>
<targetPath>${project.build.directory}</targetPath>
<file>${project.build.directory}/${project.build.finalName}.jar</file>
<destFileName>${project.build.finalName}.baz</destFileName>
</resource>
</resources>
</configuration>
</execution>
</executions>
</plugin>
The beautiful simplicity of the above code is that, as long as you specify a finalName (or even if you don't? I haven't tested that), this should work generically for any project.
Enjoy.