How can I extend drush aliases with specific variables?
The Drush rsync and drupal-directory (dd) commands already supports the functionality you desire, without the need for you to extend your alias files.  For example, to copy your files directory:

drush rsync @live:%files @dev:%files

To copy a specific folder from your temporary directory:

drush rsync @dev:%temp/foo @live:%temp

To change your working directory to the location where the devel modulr is installed:

cd $(drush dd @dev:%devel)