In addition, I discovered some javax.servlet.* classes in my JAR after running "mvn package". I believe this is caused by the GWT plugin sucking these in when it compiles my ProxyServlet. I excluded them by adding the maven-jar-plugin.
After doing this, I was able to publish my JAR with all the contents I needed to run Selenium tests against it.
Testing the GWT Library with Selenium
The module that contains the Selenium tests is a WAR project that uses war overlays, Cargo and Selenium RC. You can read about the Maven setup I use for running Selenium tests in Packaging a SOFEA Application for Distribution.
The major difference when testing a JAR (vs. a WAR), is I had to use the maven-dependency-plugin to unpack the JAR so its contents would get included in the WAR for testing. Below is the configuration I used to accomplish this:
Hopefully this will help you develop GWT libraries and run Selenium tests against them. If you have any suggestions for simplifying this configuration, please let me know.