pl/python has access to the all the modules that the normal Python interpreter would have as long as they are in the $PYTHONPATH on the server (and the user that runs the postgres service). Does import lucene work if you run it in the Python interpreter on the server?
If your module is installed somewhere else (e.g. not dist-packages etc.), then you would need to edit your /etc/postgresql/9.1/main/environment (adjust to your PostgreSQL version) file on the server and add something like PYTHONPATH='<path to your module>'.