Currently, L&Fs can only be registered through an explicit UIManager call, which
prevents widespread of user selectable L&Fs and each application have to invent its own way of specifying and finding L&F.
On the other hand, there is nice registration technique used throughout the JDK for registering services: http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#Service Provider
which can be used for the purpose of registering L&Fs as well.
If this technique is implemented, installing L&F would be a matter of placing it on the classpath (e.g. into $JRE/lib/ext) and all applications would then be able to reach all installed L&Fs the same way (UIManager API)