I construct a Mac DMG from an app thanks the Ant API of the Packager:

        <fx:deploy verbose="true" nativeBundles="dmg"
                   outdir="${basedir}/${dist.dir}"
                   outfile="${bundle.name.root}"
                   mac.app.image="${basedir}/${dist.dir}/signedimage/${application.title}.app"
                   width="100" height="100">
            <fx:application refid="com.oracle.javafx.scenebuilder"/>
        </fx:deploy>

Using the packager of 8u20 b02 it works like a charm as with b15 to b18 I get the failure below.
I didn't try with builds b03 to b14.

No base JDK. Package will use system JRE.
Building DMG package for JavaFX Scene Builder
  Config files are saved to /var/folders/2h/f4qpfbnn0n18s12l93tphzh000grx0/T/fxbundler6619595309253065927/macosx. Use them to customize package.

BUILD FAILED
build.xml:700: Error: Bundler "DMG Installer" (dmg) failed to produce a bundle.