The java.library.path can be set before or during the execution of a java app (either by setting the global environment variables of the java library path or by using a flag when running the java exec jar). Can it be set inside the manifest file of the java jar, as is done for classes with Class-Path? Sounds like a feature that should be supported, if it does not exist.
EDIT : since as stdunbar mentioned, the manifest file does not support java.library.path mappings, I've added an answer with an alternative solution to using the manifest file.