In javaws 1.4.1, the properties listed in the main jnlp file, as well as the extensions, were all set depending on the permissions in the main jnlp file, when it's first jar was loaded.
In 1.4.2, the properties for each extension jnlp file, are loaded based on the permissions granted to that extension (which is correct) but they are not set until the first jar of the extension is loaded. In this case that is too late.
If the loading of the jar in the extension is triggered by the properties set in the extensions jnlp file, the propertys are now never set, and the jar is never loaded.