There is no usable api to download nativelib resources. In a future rev of the spec we should add DownloadResource.loadNativelibResource() so that native libraries not listed in a jnlp file can be downloaded dynamically.
In the mean time, if a nativelib is listed in the jnlp file, (see 6758882 to allow them to be lazy) using downloadResource() should reccognize that the resource is a nativelib, and explode the jar file and add its location to the native library path, so they can be used.