Build a JavaFX app with Java 7u45 and deploy an application image.
Remove any "runtime" directory from the image so the Current JRE is used.

If you try to run the launcher .exe with Java 8 installed as the current JRE, this results in a dialog stating:

"This application requires a newer version of the Java runtime. Please download and install the latest Java runtime from java.com. Then restart the application."

Rebuild the application with JDK 8 (remove runtime folder again if needed). The Java 8 launcher stub is able to load the application.

Change the Current Java Runtime to 1.7 in the windows registry and then the Java8-created launcher claims "no main class" "failed to launch JVM"

(Application was compiled with target 1.7 in both cases.)