For this method you don't need to unzip the entire contents, instead you can use the Zip API to just get an single entry inside the zip and extract that directly into the user's project.  Don't even need to have a temp dir.  

inputStream = zipFile.getInputStream( new ZipEntry( "/path/to/lib.jar" ) );
// write the intput stream out to a file in user's project using
IFile libFile = project.getFile("/path/to/lib.jar");
libFile.create( inputStream )