Yes, very possible I do this all the time. Also I use Jetty (J2EE Preview) in Eclipse WTP it's very fast and works well, on to the weaving though. Make a weave.xml file that ant can handle like this:
<?xml version="1.0"?>
<project name="MyProject" default="weaving" basedir=".">
<property environment="env"/>
<path id="weavingclasspath">
<fileset dir="${basedir}\\web\\WEB-INF\\lib">
<include name="**/*.jar"/>
</fileset>
<fileset dir="${basedir}\\antlib">
<include name="**/*.jar"/>
</fileset>
<pathelement location="classes"/>
<dirset dir="${basedir}\\build">
<include name="**/classes"/>
</dirset>
</path>
<target name="define.task" description="New task definition for EclipseLink static weaving" >
<taskdef name="weave" classname="org.eclipse.persistence.tools.weaving.jpa.StaticWeaveAntTask">
<classpath refid="weavingclasspath"/>
</taskdef>
</target>
<target name="weaving" description="perform weaving" depends="define.task">
<weave loglevel="WARNING" source="${basedir}\\build\\classes\\" target="${basedir}\\build\\classes\\">
<classpath refid="weavingclasspath"/>
</weave>
</target>
</project>
Put this in your project root and execute it as an Ant Task. Make sure you refresh your project when done since all the .class files for your entities will be weaved. I actually link this task to the eclipse build process so it's automatic. You might need to modify some of the file paths in the ant script for your project to get it to work, but you should get the idea. My antlib has servlet.jar in it (I don't remember why) and my WEB-INF/lib has all my web project jars as well as eclipselink.