There's a limitation in islandora pathauto that prevents it from being extended to use values of datastream elements in the url aliases.
This limitation is caused by a workaround for ISLANDORA-1073 - i.e. when the label is updated, we have to mock up a fake object to get the tokens from because in all the applicable islandora hooks, $object->label still has the old value. Because of this, datastreams are not accessible when an update to the object triggers an update to the aliases.
If we make a more thorough fake object, then it will be trivial to extend islandora_pathauto and make tokens (ie URL components) based on datastream values.