This works:

<?xml version="1.0" encoding="UTF-8"?>
<project name="JavaFxDemo Build" default="-post-jfx-deploy-release"
    basedir="." xmlns:fx="javafx:com.sun.javafx.tools.ant">

<description>This procedure will compile/build the JavaFxDemo source</description>

<!-- Set global properties for this build -->
<property name="jdk.home" value="D:/jdk1.8.0"/>
<property name="src" location="src"/>
<property name="build" location="build"/>
<property name="Release" location="Release"/>
<!-- Setup properties for JavaFX packaging -->
    <property name="javafx.tools.ant.jar" location="${jdk.home}/lib/ant-javafx.jar"/>
    <property name="application.title" value="JavaFxDemo"/>
    <property name="application.vendor" value="GDG Space Coast"/>
    <property name="application.version" value="1.0"/>
    <property name="javafx.main.class" value="com.gdgspacecoast.javafxdemo.JavaFxDemo"/>

<!-- Set the project class path -->
<path id="project.class.path">
<pathelement location="${build}"/>
<pathelement location="${jdk.home}/jre/lib/rt.jar"/>
</path>

<!-- Delete the ${build} directory tree -->
<target name="clean" description="Clean up tasks" >
<delete dir="${build}"/>
</target>

<!-- Create the build and distribution directory structure -->
<target name="init" depends="clean" description="Initialization tasks">
<tstamp/>
<mkdir dir="${build}"/>

<copy todir="${build}/com/gdgspacecoast/javafxdemo">
<fileset dir="${src}/com/gdgspacecoast/javafxdemo" />
</copy>
</target>

<!-- Compile the java code from ${src} into ${build} -->
    <echo message="Using Java version ${ant.java.version}."/>
<target name="compile" depends="init" description="Compile the source " >
<javac bootclasspathref="project.class.path"
srcdir="${src}"
debug="true"
debuglevel="lines,vars,source"
destdir="${build}"
nowarn="true"
source="1.7"
target="1.7"
includeantruntime="false"
fork="yes">
</javac>
</target>

<!-- Package the classes and resources into a self-contained app bundle -->
<target name="-post-jfx-deploy-release" depends="compile"
description="Generate the native bundles in release mode">
<delete dir="${Release}" />
<mkdir dir="${Release}" />
<taskdef resource="com/sun/javafx/tools/ant/antlib.xml" uri="javafx:com.sun.javafx.tools.ant"
classpath=".:${javafx.tools.ant.jar}" />

<fx:application name="JavaFxDemo" id="JavaFxDemo"
mainClass="${javafx.main.class}" version="${application.version}" />

<!-- Create doubleclickable jar file with embedded launcher -->
<fx:jar destfile="${Release}/JavaFxDemo.jar">
<fileset dir="${build}" />
<fx:application refId="JavaFxDemo" />
<manifest>
<attribute name="Implementation-Vendor" value="${application.vendor}" />
<attribute name="Implementation-Title" value="${application.title}" />
<attribute name="Implementation-Version" value="${application.version}" />
</manifest>

<!-- Setup the classpath for the generated .jar here -->
<fx:resources>
<fx:fileset type="jar" requiredFor="startup"
dir="${Release}" includes="JavaFxDemo.jar" />
</fx:resources>
</fx:jar>

<fx:deploy width="1000" height="730" nativeBundles="image"
outdir="${Release}" outfile="${application.title}">

<fx:application refId="JavaFxDemo" />

<!-- Copies the actual library .jars to the bundle here -->
<fx:resources>
<fx:fileset type="jar" requiredFor="startup"
dir="${Release}" includes="JavaFxDemo.jar" />
</fx:resources>

<fx:info title="${application.title}" vendor="${application.vendor}" />
</fx:deploy>
</target>

</project>


But when I try to select a different runtime like this:

<?xml version="1.0" encoding="UTF-8"?>
<project name="JavaFxDemo Build" default="-post-jfx-deploy-release"
    basedir="." xmlns:fx="javafx:com.sun.javafx.tools.ant">

<description>This procedure will compile/build the JavaFxDemo source</description>

<!-- Set global properties for this build -->
<property name="jdk.home" value="D:/jdk1.8.0"/>
<property name="jdk.7u25" value="D:/jdk1.7.0" />
<property name="src" location="src"/>
<property name="build" location="build"/>
<property name="Release" location="Release"/>
<!-- Setup properties for JavaFX packaging -->
    <property name="javafx.tools.ant.jar" location="${jdk.home}/lib/ant-javafx.jar"/>
    <property name="application.title" value="JavaFxDemo"/>
    <property name="application.vendor" value="GDG Space Coast"/>
    <property name="application.version" value="1.0"/>
    <property name="javafx.main.class" value="com.gdgspacecoast.javafxdemo.JavaFxDemo"/>

<!-- Set the project class path -->
<path id="project.class.path">
<pathelement location="${build}"/>
<pathelement location="${jdk.home}/jre/lib/rt.jar"/>
</path>

<!-- Delete the ${build} directory tree -->
<target name="clean" description="Clean up tasks" >
<delete dir="${build}"/>
</target>

<!-- Create the build and distribution directory structure -->
<target name="init" depends="clean" description="Initialization tasks">
<tstamp/>
<mkdir dir="${build}"/>

<copy todir="${build}/com/gdgspacecoast/javafxdemo">
<fileset dir="${src}/com/gdgspacecoast/javafxdemo" />
</copy>
</target>

<!-- Compile the java code from ${src} into ${build} -->
    <echo message="Using Java version ${ant.java.version}."/>
<target name="compile" depends="init" description="Compile the source " >
<javac bootclasspathref="project.class.path"
srcdir="${src}"
debug="true"
debuglevel="lines,vars,source"
destdir="${build}"
nowarn="true"
source="1.7"
target="1.7"
includeantruntime="false"
fork="yes">
</javac>
</target>

<!-- Package the classes and resources into a self-contained app bundle -->
<target name="-post-jfx-deploy-release" depends="compile"
description="Generate the native bundles in release mode">
<delete dir="${Release}" />
<mkdir dir="${Release}" />
<taskdef resource="com/sun/javafx/tools/ant/antlib.xml" uri="javafx:com.sun.javafx.tools.ant"
classpath=".:${javafx.tools.ant.jar}" />

<fx:application name="JavaFxDemo" id="JavaFxDemo"
mainClass="${javafx.main.class}" version="${application.version}" />

<!-- Create doubleclickable jar file with embedded launcher -->
<fx:jar destfile="${Release}/JavaFxDemo.jar">
<fileset dir="${build}" />
<fx:application refId="JavaFxDemo" />
<manifest>
<attribute name="Implementation-Vendor" value="${application.vendor}" />
<attribute name="Implementation-Title" value="${application.title}" />
<attribute name="Implementation-Version" value="${application.version}" />
</manifest>

<!-- Setup the classpath for the generated .jar here -->
<fx:resources>
<fx:fileset type="jar" requiredFor="startup"
dir="${Release}" includes="JavaFxDemo.jar" />
</fx:resources>
</fx:jar>

<fx:deploy width="1000" height="730" nativeBundles="image"
outdir="${Release}" outfile="${application.title}">

<fx:application refId="JavaFxDemo" />

<!-- Copies the actual library .jars to the bundle here -->
<fx:resources>
<fx:fileset type="jar" requiredFor="startup"
dir="${Release}" includes="JavaFxDemo.jar" />
</fx:resources>

<fx:platform basedir="${jdk.7u25}" />

<fx:info title="${application.title}" vendor="${application.vendor}" />
</fx:deploy>
</target>

</project>

The application fails to start with "no main class" and "Failed to Launch JVM". Removing the <fx:platform basedir="${jdk.7u25}" /> line resolves the issue, but obviously looses the feature's functionality (RT-24766).

Using JDK 1.7.0_40-ea-b38 to do the packaging and selecting to bundle JDK 7u25 works just fine!