Na verdade não basta copiar os arquivos para jre/lib/ext. Se realmente quer outras LAFs, você precisa criar um arquivo swing.properties (é necessário ler o fonte de UIManager.java para saber como criar e editar esse arquivo.) Veja o fonte de getInstalledLookAndFeels para entender porque é que ele só retorna essas 3 LAFs.
/**
* Returns an array of objects that provide some information about the
* <code>LookAndFeel</code> implementations that have been installed with this
* software development kit. The <code>LookAndFeel</code> info objects can
* used by an application to construct a menu of look and feel options for
* the user or to set the look and feel at start up time. Note that
* we do not return the <code>LookAndFeel</code> classes themselves here to
* avoid the cost of unnecessarily loading them.
* <p>
* Given a <code>LookAndFeelInfo</code> object one can set the current
* look and feel like this:
* <pre>
* UIManager.setLookAndFeel(info.getClassName());
* </pre>
* @return an array of <code>LookAndFeelInfo</code> objects
*
* @see #setLookAndFeel
*/
public static LookAndFeelInfo[] getInstalledLookAndFeels() {
maybeInitialize();
LookAndFeelInfo[] ilafs = installedLAFs;
LookAndFeelInfo[] rv = new LookAndFeelInfo[ilafs.length];
System.arraycopy(ilafs, 0, rv, 0, ilafs.length);
return rv;
}
/**
* The default value of <code>installedLAFS</code> is used when no
* swing.properties
* file is available or if the file doesn't contain a "swing.installedlafs"
* property.
*
* @see #initializeInstalledLAFs
*/
private static LookAndFeelInfo[] installedLAFs = {
new LookAndFeelInfo("Metal", "javax.swing.plaf.metal.MetalLookAndFeel"),
new LookAndFeelInfo("CDE/Motif", "com.sun.java.swing.plaf.motif.MotifLookAndFeel"),
new LookAndFeelInfo("Windows", "com.sun.java.swing.plaf.windows.WindowsLookAndFeel")
};