You could use Ant's <record>
task (http://ant.apache.org/manual/Tasks/recorder.html) to get verbose logging to a file. If this task is defined early in the build file, you should get logging for all build tasks. You could also start and stop the recorder anywhere in your build file. This could, for example, allow you to not log the output of some task that you do not want to see in the log file.
Here's an example of a simple build file that uses the <record>
task:
<?xml version="1.0" encoding="UTF-8"?>
<project default="all" basedir=".">
<record name="build.log" loglevel="verbose" action="start" />
<target name="all">
<path id="all.files">
<fileset dir="." includes="**/*" />
</path>
<property name="files" refid="all.files" />
<echo level="verbose">files=${files}</echo>
</target>
</project>