1.4.2 (and probably the 1.4.x branch) of Connector/J is not compatible with Google App Engine because it uses a restricted class. Here is the error produced by App Engine:
*_java.lang.management.ManagementFactory is a restricted class. Please see the Google App Engine developer's guide for more details
The 1.3.x branch of Connector/J does not have this problem and works fine on App Engine. There are a lot of Google App Engine users, so it would be best to remove the 1.4.x branch's dependency on this class.