I committed revision 1356860 to trunk.
This first iteration of this tool is limited in usage. Currently it is not configurable. Once it is on the classpath, it starts running. When you take it off the classpath, it stops running. It is also depended upon runtime (ie: javaagent or container enhancement) enhancement.
I modified a number of things from the original patch to make it more usable... When the code first gets loaded up you'll see this message so a user is notified when it is running. Also, when the statistics are dumped you'll see a message along these lines.
I'll also note that I modified the pom to spit out another jar file that allows this tool to work in a WebSphere environment.
 3094 fetchStatisticPU INFO [main] openjpa.Runtime - The FetchStatisticsAuxEnhancer has been loaded and is tracking persistent field usage.
 4226 fetchStatisticPU INFO [Thread-5] openjpa.Runtime - Successfully collected fetch statistics from Entities [org.apache.openjpa.enhance.stats.AEntity,..., org.apache.openjpa.enhance.stats.EEntity]. The following fields are FetchType.EAGER and were never fetched [ total 10 ] :