Drupal callback :: Less CSS file with variable_get()'s
It may appear like a file to the browser if you were to directly request that path in the browser, but drupal_add_css() and the LESS module deal with the file system directly and does not attempt to resolve file paths through Drupal's menu system.

drupal_add_css() will not add files that do not exist on the file system.

If you are attempting to have programmatically assigned variables within a .less file I would suggest checking out the Variables functionality that is available within the latest version of the LESS CSS Preprocessor module, documentation here: https://drupal.org/node/2139603

If you are truly trying to create a dynamically composed .less file, you first need to create the file in the files directory of the drupal install and then add that file with drupal_add_css().