Basics: Ant already has an output property. So you can just add the format option and output it to a file specified as an output property on the apply element. in an h5bp project this will output "output.xml" to the root of your project.
<apply dir="${dir.source}/${dir.css}"
executable="java" parallel="true"
failonerror="true" output="output.xml">
<fileset dir="./${dir.source}/">
<include name="**/${dir.css}/*.css"/>
<exclude name="**/*.min.css"/>
<exclude name="**/${dir.publish}/"/>
</fileset>
<arg value="-jar" />
<arg path="./${dir.build.tools}/${tool.rhino}" />
<arg path="./${dir.build.tools}/${tool.csslint}" />
<arg value="--format=lint-xml" />
<srcfile/>
</apply>