This configuration worked for me to compile Kotlin onto Android from the command line. My ant-fu is not great, but hopefully this helps someone else who struggled with it.

custom_rules.xml

<?xml version="1.0" encoding="UTF-8"?>
<project name="kotlin_rules">
    <property environment="env"/>
    <typedef resource="org/jetbrains/kotlin/ant/antlib.xml" classpath="${env.KOTLIN_HOME}/lib/kotlin-ant.jar"/>
    <target name="-post-compile">
        <path id="project.javac.classpath">
            <path refid="project.all.jars.path" />
            <path refid="tested.project.classpath" />
            <path path="${java.compiler.classpath}" />
            <path path="${project.target.android.jar}"/>
            <path path="${out.classes.absolute.dir}" />
        </path>
        <kotlinc src="src" output="bin/classes"
            classpathref="project.javac.classpath" />
    </target>
</project>