The new Java Plug-In supports cross-platform selection of a particular JRE version on which to execute a particular applet via a new parameter to the applet, object and embed tags:

  <PARAM name="java_version" value="1.5*">

The value may be any version string supported by Java Web Start in the JNLP file format.

This functionality needs to be documented publicly. Note that this capability does not imply the same level of specification as a public API; it is more of an implementation detail, similar to "codebase_lookup", which is documented at http://java.sun.com/javase/6/docs/technotes/guides/plugin/developer_guide/special_attributes.html .
Engineering specification for feature:

http://j2se.sfbay.sun.com/web/bin/view/Deployment/NewPlugInJavaVersionSpec